For the past five years, at the very least, there has been something in my life I have tried to deny. I have hidden it from others, or at least I thought I had – they would tell a different story – and I have tried to deny it to myself. But if I'm honest, it has always been there in one way or another, born in my adolescence and nurtured to strength by middle age and by motherhood. I can say two things with equal certainty: the first is that I cannot remember with any real clarity what it feels like not to worry, sometimes obsessively, about my health; the second is that acknowledging this causes me great shame. If the conscious, unspoken refrain in my head has been, at fairly frequent intervals, the absolute certainty of the end of my functioning life as I know it, its counterpoint has been to reproach and chastise myself for such self-indulgent introspection.

Like most people of my age – I will be 40 on my next birthday – I have witnessed and experienced loss. I have listened in horror and sympathy to stories of strong, capable women battling against breast cancer, losing their hair and sometimes their breasts, remaining strong for their children, and often emerging triumphant. My vibrant, wonderful friend and literary agent died of liver cancer within a week of diagnosis, leaving behind her beloved young daughter. I watched my mother-in-law hand herself over to the demands of her husband's disease, multiple sclerosis, which, over the five-year period while my three children were growing strong and healthy, was cutting him down, immobilising his 6ft 7in frame, and eventually leaving him utterly dependent on her. What right have I to complain in the face of such real suffering and such bravery? If you find yourself thinking this as I tell the story of my health anxiety, then I can only say I have thought it a million times myself.

Recently Brian Dillon published to great acclaim Tormented Hope: Nine Hypochondriac Lives, in which he provides elegant, empathic biographies of the world's greatest hypochondriacs: Boswell, Brontë, Florence Nightingale, Proust, Warhol and more. The book, accompanied by an incisive introduction on the condition, is not only a mini-masterpiece, but for the so-called "hypochondriac" or "health anxious" like me a source of great comfort and reassurance.

Health anxiety, as hypochondria is called now, is on the increase, another product of our privileged but stressful times. Up to one in 10 of us suffers from some kind of anxiety problem during our lifetime, and GPs are now seeing more cases in which this is manifested in health. And yet while people nod and agree in sympathy whenever the word is spoken, hardly anybody will own up to it publicly. If they do own up, it is usually in the form of some kind of joke on themselves, a way of saying "Aren't I funny?" rather than "Aren't I mad?". Dillon, on the other hand, a self-confessed reformed hypochondriac, is honest about its exhausting downward spiral: "However the suspicion may have insinuated itself," he writes, "in the days that follow it seems to sharpen in your mind. Your symptoms appear to point to a specific illness: it is the disease, perhaps, that you have feared all your life, or in recent years; the disease of which a parent died. Your first fears begin to condense into certainties, no less fearful. You feel compelled to research your disease."

The novelist William Boyd identifies the human condition as walking a tightrope between happiness and unhappiness. Given I spend so much time worrying about illness, paradoxically when the anxiety subsides not a night goes by when I do not count my blessings. As glib as it sounds, I have always appreciated the state of happiness, and never more so since becoming a mother, easily the most transforming, rewarding experience of my life. For me, health anxiety is a compartment of my life – a dark cell, really – usually totally separate from my day-to-day self.

But I have found that now, more than ever, I search for reassurance. I want so much to be around to see my children grow up. As a child, I would pray to God – extra politely for maximum effect – that He keep my parents and sister safe. Now, 30 years on, I do the same for my family, with the same infantile construction: "Dear God, My name is Louise, I have three children and a husband… Please keep us safe." I am not especially religious and I am certainly not deluded enough to think that God will save my family and me from disease. What I want, though, is a defence to uncertainty. I want not to die early or become an incapacitated shadow of my former self, a dribbling burden on my husband and my daughters. This is the play into which I cast myself as the leading part. It's never the starting point, but it's the destination of the worry. It's where the imagined – or "real" – symptoms are headed. As Brian Dillon says: "What sort of idiot spends the best part of a decade convinced of his own imminent collapse?" Well, the answer to that one is me.

It began, I think, when I was 16 and studying for my O-levels. It's almost a joke now to recognise that my Saturday job was in a pharmacy. I was constantly hanging around the pharmacist, watching him measure out the pills and mix up the medicine, as Bob Dylan put it. ("Mummy's in the basement mixing up the medicine," my children often sing now, an in-joke taught to them by their father.) When punters came in with their urine samples, I'd carry them to the pharmacist's pill-lined lair as if I were Florence Nightingale herself. One Saturday at the shop I collapsed. My face went numb and my arms and hands went tingly. I was drowsy, and I heard the pharmacist say to one of the other assistants: "Call a doctor – I think she's having a stroke." I was taken home and put to bed. Our GP diagnosed hyperventilation. It happened again, about four years later when I was at Pisa airport. I was tired and hadn't eaten. Around this time, I think, migraines started. About a year later, when I was particularly unhappy in a new job, I had another terrifying episode of numbness. I was referred to a neurologist and I had a brain scan. My brain was fine. He tested my reflexes, I assume for multiple sclerosis. He gave me the all-clear but told me to come off the contraceptive pill because of a "predisposition towards strokes". I skipped off content I was fine, but in fact I cannot say that that was that. In the back of my mind I convinced myself I had MS, although you'd think it would be a stroke I would have feared more. But still, I was young and only had myself to worry about. (The neurologist's comment came back to haunt me this year, however. I went to my GP to ask if the notes could be called back for him to go through with me. He very patiently did this, once again ruling out any cause for concern.)

Throughout my 20s I avoided red wine and caffeine. My friends say I was hilarious back then, constantly niggling them with nonsense, but I don't remember talking about my health.

Over the past five years, since the birth of my three children, I estimate that I have been to the doctor's more times than in the preceding two decades. Unlike some hypochondriacs, there is some part of me that recognises the neurosis, but I find myself in a loop; that talking myself out of a surgery visit might be seen as an act of hubris for which I'll be punished. It's a lose-lose situation. There is no logic here.

In 2004, soon after the birth of my first baby, I went to the GP with a large bump on the back of my skull. I was convinced I had cancer of the skull (I don't even know if this condition exists. I was probably too tired with a newborn to check it out on the internet). She acknowledged that its size was unusual and sent me for X-rays. I was fine. It was the shape of my head. I suspect it might be the legacy of falling down the stairs when I was 21 and drunk at a party.

A couple of years ago I got an infection in my knee. A swelling also came up in my groin. I jumped to the obvious conclusion – melanoma. My mother had a melanoma removed from her leg caused by too much sunbathing, and when we were children in the sun-worshipping 70s she would roast us to a crisp. It was my turn. I saw an out-of-hours GP who gave me antibiotics, but described it as "an unusual lesion". Hours were lost on the internet after hearing that phrase. I did correctly diagnosis myself as having cellulitis, a bacterial infection probably from a dodgy leg wax, but it also took a lot of work from my current GP to convince me I was fine.

Following this, there was a lump on my waist. That took two visits, during which I had prepared myself to hop up onto the couch to hear the worst. (I did later wonder if the innocent lump had been caused by the fact that I refused to admit my 7 for All Mankind jeans were now painfully too small for me.)

Recently, back from a holiday in the south of France, the first holiday where my children were old enough to let me sit in the sun for about 15 minutes flat, I once again became convinced I had a melanoma. A black spot appeared on the back of my leg. It looked a bit inflamed. Back I went to my old friend the internet, where I pored over pictures of melanomas. It was a black dot. "It's a bite," said my husband. It was not a bite. Fuelled by stories of lives shortened by missed moles (often on the back of the leg), I rushed to the GP, who provided instant reassurance. "Are you quite sure?" I asked. I look at it now and it's a nice little freckle. It is shameful. There is no other word for it.

All these things melted away with reassuring words from my GP. Worry over my brain, or rather worry over MS, is a more complicated fear, and one that has taken a more permanent hold. In the last couple of years, there have been two more episodes of numbness when I thought I would faint. Both were first thing in the morning and both when I was up without breakfast. One was when I was pregnant and, for a second, I slurred a word. "It's your blood sugar," my husband said. "For God's sake, eat breakfast." I told the midwife and she looked anxious. She told me to talk to the doctor. I didn't because I was too frightened. With a false logic, I concluded that I wanted to experience the happiness of my unborn child for as long as I could before I finally found out I would be an invalid.

Recently I found I was getting a tingling sensation in the ends of my fingers, mostly when I was driving. I went on the internet – as usual – this time as a way of trying to banish MS worries. I diagnosed myself with Raynaud's syndrome, a circulatory disorder (I also have chilblains, which did surprise the GP, and my hands are always cold). The relief of thinking it was not MS did not last long. Why had this MS fear taken such a hold of me now?

A couple of months ago my friend, the writer Amy Jenkins, came over for a chat. As our children played happily, I fought off tears and voiced my concerns to her while my husband was out of the room. His relationship with my MS neurosis is justifiably complicated: "Go and get yourself checked out," she said. "You'll have 10 days of worry while you wait for the results rather than 10 years of worrying about nothing." I nodded solemnly. A week later she asked if I'd done it. "No, no," I said, "I'm fine." In truth I was terrified.

The Greek word "hypochondria" roughly translates as "below the ribcage". Over the past 3,000 years it was used to explain indigestion, then melancholia, then neurosis and then, finally, "a misplaced fear of illness based on misinterpretation of bodily symptoms". Statistics have been bandied about by doctors: the equivalent of one day a week of surgery time lost to these perfectly healthy people; up to 13% of us worrying about our health when we might not have done in the past.

In Tormented Hope, all but one of the famous nine seem to prove the cliché that hypochondria tends to be a "disease of the learned"; that its sufferers are usually people caught between the prosaic nature of the real world and the crushing burden of their creativity. Dillon stops short of saying it himself, but it proves the 18th-century theory that it is an imagined sickness born of creative anguish. Freud, on the other hand, believed that it merely masked a more deep-seated neurosis such as homosexuality.

Contemporary ideas about hypochondria include these: as cave persons, we were hard-wired to worry about threat. John Naish, in his book The Hypochondriac's Handbook: A Disease for Every Occasion, An Illness for Every Symptom, points out how modern sanitation and medicine have eliminated the old dangers, but modern civilisation has given us more time, money and energy to fixate on illness. There has been a huge drop in mortal illness in the western world, but a massive increase in new diagnoses. As these new "illnesses" emerge, they are over-reported and given disproportionate significance.

There have been two other major shifts in society. The first is the rise of the internet, which has spawned "cyberchondria". Health is now the second-most popular internet search topic after pornography. Millions of people tap symptoms and diseases into Google and wait for some dreadful outcome. I am an aficionado of these sites (my favourite is the NHS site, patients.co.uk). We terrify ourselves as we read information we do not understand and use to justify our worst fears.

The second change is the role of the GP. As one told me recently: "People don't trust their GPs any more. We haven't the time to give patients what they need, and it's resulted in a breakdown of trust. They go on the internet themselves."

In my efforts to help myself, I seized upon Health Anxiety – A Self-Help Guide, published on the internet (of course) and written by four clinical psychologists from Newcastle, North Tyneside & Northumberland Mental Health NHS Trust. The guide is how I imagine do-it-yourself cognitive behavioural therapy works. Patients are invited to keep diaries of their preoccupations and imagined symptoms and then told to counteract them with realistic and rational thinking. Symptoms such as throbbing headaches and tingling fingers and toes, the guide spelled out, can actually be caused by the health anxiety, because the mind talks the body into a fearful fight-or-flight state. Because the hypochondriac is so hyper-aware of his or her body, these sensations are blown out of all proportion and become part of a spiral of panic.

The guide was a Eureka moment for me. I had no idea symptoms could effectively be self-generated. "There are many reasons why someone worries too much about their health," says Lorna Cameron, one of its authors. "You may be going through a particularly stressful period of your life. There may have been illness or death in your family, or a family member may have worried a lot about your health when you were young.

"Also, a lot of anxiety can relate to a sense of increased responsibility. If it is your belief that you have an absolute duty and responsibility to care for somebody, then you get anxious that you will not be able to do that. Every case is different, but there are underlying themes. Witnessing a misdiagnosis in the past is one. What we try to do is work out how the patient reached where they are, what their underlying beliefs about illness are."

Brian Dillon explained to me how it was for him: "From the time I was 10 my mother was very ill with a rare autoimmune disorder called scleroderma, from which she died in 1985, when I was 16. She also suffered from depression from the time I was three, so illness seemed very much part of our lives. My father died suddenly when I was 21, and it was really then that my fears escalated: I had constant scares – mostly cancer and heart disease – in my 20s, and things only improved after I was diagnosed with depression at 28 and properly addressed everything I'd been terrified about for years. I really believed that illness was just what happened when you grew up, and then the way I expressed my later anxiety and depression through imaginary or psychosomatic symptoms. My sense is that we miss the fundamental questions – about our bodies, our futures, our relationships, about death – if we simply think of 'health anxiety' as an anxiety disorder that can easily be treated with CBT and antidepressants."

How easy it is to see an obvious explanation for someone else's health anxiety. For myself, though, where to start? In the book Bedside Stories, Confessions of a Junior Doctor, based on a column that ran in the Guardian, Michael Foxton tells the story of a night working in Accident and Emergency. A terrified, screaming mother runs in with her blue infant. An entire A&E department converges on the child. And in the middle of it all, another woman who has waited eight hours with a sprained ankle tries to block Foxton's path. Foxton levers her to the wall to get to the child. The child dies. Afterwards Foxton asks his consultant: "Why can't people just get over the idea that we have some kind of open-ended commitment to their flakiest conditions?"

For all of us hypochondriacs out there, there are also friends and family who love and care for us and on whom our fear makes unfair demands. Nobody with health anxiety keeps it from others. It's impossible. My husband has always been fairly calm about it, but my growing preoccupation had begun to cause him difficulty, too. "For me, to keep you calm about these things has been a prerequisite of dealing with them. It's only with the MS worry that I've had to conclude that you have had some kind of condition. I suppose I felt it was my patch. I feel that the damage it has already done to my family can't continue, and maybe that I should have the monopoly on suffering from the thing. The consequences are that I rule it out, and I deny you what might really be a way in which you mourn my father. Maybe it is a good thing that I had to confront your way of confronting it."

At this point I had no idea if my symptoms were real or psychosomatic – a response, perhaps, to my family situation. Last February my father-in-law finally died from MS. My husband was grief-stricken long before this moment. Although my father-in-law's demise was slow and shocking, his death was unexpected. What amazes me is that until now I have never made the connection between this and my growing neurosis.

Worn down by anxiety, I wrote to my GP. The letter was full of apologies and self-flagellation. I told him I knew I was behaving like a "lunatic". A week later a reply arrived: "I am happy to see you," he wrote. "I promise not to call you a lunatic and hopefully I can help you put things in perspective… I certainly can't claim to be an expert in health anxiety other than that I see people with it every day and consider it to be quite normal. Can I suggest that you make a double appointment, which will give us a little more time."

Three days ago I went to the surgery for my double appointment. It all came tumbling out; the first episode of numbness when I was 16; the most frightening one two years ago when my tongue was numb; my father-in-law's decline; my terror of leaving my children motherless.

He looked at me and said this: "There is nobody in the world who, hearing your situation with the various elements, would not understand how you feel. It is normal. It is natural for you to worry about leaving your children. It is evolutionary, otherwise we'd all be bungee-jumping and putting our lives at risk.

"Hypochondriacs come into my room looking for attention, or sympathy, or pity, and it is a mental health issue. People are in here all the time wanting reassurance. It is what we do. We are a filter. For you, there is something tangible too."

He was right: small, tangible incidents spawning worry and yet more worry, further intensified by family suffering, growing older, the weight of parental responsibility and responsibility to my husband not to get the same illness that robbed him of his father. And then came the words: "I would like to refer you to a neurologist."

I felt the old fear. My face flushed with panic. It was the Spike Milligan joke (he had inscribed on his tombstone: "I told you I was ill").

"I think there is nothing to worry about. I think what you suffer from is the aura before a migraine," he said, which is basically what the first neurologist said 15 years ago. "And when you get numbness in your fingers when you drive, it is the compression of the nerves in your wrist – very common. But I know that you will not believe me until you hear it from an expert."

I don't want to know, I told him. "Don't you think you need to stop worrying now?" he asked. "The consultant will examine you and then will tell you to go away and stop wasting their time."

And so I have an appointment next month. I suppose there is irony in the fact that I am not a hypochondriac after all, that I am off to hospital. "You seem very calm about it," my husband observed. I don't know about calm, but as I write this, I am trying to concentrate on the freedom I hope will come from being told that I never have to think about it again. And that is a promise.