This time last year I was keeping a diary, the writing small and meticulously neat. In it, I recorded everything that happened each day. Not in the outside world but to me, inside my own body. It was the only thing that seemed important enough to set down. “Lots of muscle spasms today,” I wrote on 20 February, pen pressed hard to the page. “Aches and twinges on my right side, getting more acute in the evening. Left leg still weak.” At the back of the diary were lists: girls’ names, US states, English counties – all written out alphabetically. While my days were spent obsessively monitoring my physical symptoms, the lists were getting me through the silent winter nights. Putting my brain to work on “Alabama, Alaska, Arizona, Arkansas…” was a diversion tactic to stave off the worst of the panic.

It was two miscarriages and their medical investigations that turned my mind inward until I could do nothing but mentally scan my body, looking for trouble. I was told I had endometriosis and an underactive thyroid, and this completely wrong-footed me. If my body had kept this from me, what else was it harbouring in its darkest corners? OK, I’d been watching a lot of Game of Thrones, but I felt like an unpopular queen in a court of traitors. My body had become a thing apart from me, capable of keeping secrets.

Adding to my paranoia was the fact that I’d written about miscarriage and madness in a novel before I got pregnant. Had I brought this on myself, or were the gods punishing me? (“You wanted some good material, love? Try this.”) I’d been afraid before – mainly in airports, waiting to board – but that was a price I was occasionally willing to pay for some sun.

This fear, though, didn’t slink away on landing; it was with me all the time. Nothing felt benign any more, not even a sunny morning at home with my dogs. I couldn’t read, I couldn’t write, I couldn’t sit still, I couldn’t be on my own. One evening, the room around me shifted like a glitch in the matrix and I wondered if any of it was real. “I just want to get back to myself,” I wailed to my husband, who wasn’t much less frightened than me. “But what if I can’t?”

Danger stalked me everywhere. Every time I turned on the TV, I was besieged by hospital documentaries. Characters in books were struck down with horrible diseases. I couldn’t even go for a nice walk – once a reliable cure-all – because I was so frightened I had multiple sclerosis that I dreaded every step.

There was a really bad day when I put on some music and danced on the spot, to prove to myself that my brain was still talking to my limbs. I sobbed with fear as I did it, the dogs looking on with concerned bemusement, and told myself that this would be funny one day. And it is funny. And it isn’t, because I can still remember how much nerve it took to do it, even as I offer it up as a cheap anecdote.

Health anxiety is an epidemic estimated to cost the NHS £420m every year. While hypochondria is an age-old affliction (Charles Darwin and Marcel Proust had it), we are now mercilessly assisted by the internet. I had hoped “Dr Google” would throw up some irrefutable proof that I didn’t have whichever disease I was petrified of that day. Instead, I discovered a raft of new symptoms that my body cunningly reproduced within 24 hours. The single best thing I did in the quest to feel normal again was to keep a promise to my mum to “stop looking at those bloody websites”.

My real flesh-and-blood GP, when I went to see her, was great – sensible and kind. I hate to think of anxious people going to those Hollywood-style doctors who dish out medication like sweets, because I wanted all the drugs I could get my hands on then – to soothe my fractious body and feel as little as possible. And there was a lot to numb: bone aches, pins and needles, shooting pains, dizziness, palpitations, muscle spasms, body jolts… My shoulder muscles were so rock-hard with tension that I couldn’t lie on either side.

My GP agreed to prescribe a week’s worth of sleeping pills because not sleeping when I was used to nine hours was making me feel even more mad: wired, disordered and utterly wrung-out from all the adrenaline that was burning around my system like battery acid. That night, I took a single pill and dropped fathoms-deep into a dark, blissful slumber that went some way to calming my hair-trigger nervous system.

The deal with the sleeping pills and antidepressants I’d argued for and got, was that I also signed up for some cognitive behavioural therapy. Face-to-face sessions on the NHS came with a waiting list of four to six months, but if I was willing to try phone CBT, then I only had to wait a few weeks.

It was my counsellor, Sam, who gently suggested I give up the diary. “I think it might be making you obsess more,” she said. Another piece of “homework” was to try to stop asking for reassurance. It turned out to be just like any other craving: if I held off for a few minutes, the urge ebbed away. When I managed a whole day without asking my husband if I was walking oddly, I felt as though a fragment of my old, equable self had been restored to me. Mindfulness, though I’d previously filed it in the same drawer as chakras and homeopathy, was helpful, too.

My epiphany was understanding that anxiety wasn’t me, it was something happening to me. Instead of being consumed by it, I managed to hollow out a crucial inch of space between it and me. I’d been out in the storm and now I was watching it through the window. Instead of fighting the fear tooth and nail, I practised a rueful roll of the eyes: “Oh, here’s that old devil anxiety again, up to his usual tricks.” And slowly, slowly, that’s what it became: a series of threadbare illusions I could see right through.

A year ago, I couldn’t wait for 2018. If I got there in one piece, it meant I’d been wrong about being ill. But I see now that a bigger fear lurked beneath the hypochondria. What if my old sleep-loving, fast-walking self had abandoned me for ever? What if I could no longer be a writer because my imagination had become a dangerous place to be? And yet here I am, writing about it in the past tense. I still occasionally feel the anxiety sidle up, clammy fingers fumbling for a way in, but I’m wise to it now. Or wiser, anyway. I know it’s time to go out in the fresh air, arrange to see people, turn off my laptop. I am here, chastened but well.

How to cope with anxiety

Remember that anxiety itself can create a myriad of frightening physical symptoms. They’re horrible but they won’t hurt you.

If your GP tells you to pull yourself together, go to a different one.

Try not to feel guilty for being a nuisance or because other people really do have the illnesses you’re worried about. You can’t help it and you’re doing your best to put yourself back together.

Turn sabotaging thoughts into hard evidence by writing down everything that supports your self-diagnosis and everything that doesn’t. The second column forces you to apply some objective logic.

Avoid alcohol and caffeine. The jittery effects of too much coffee should make it an obvious no-no, but alcohol is tempting for its ability to loosen tense muscles. The hungover paranoia the next day is just not worth it, though.

Get out in the world. If you work from home, go to a co-working space at least once a week, just to be among other people.

Talk to the people you trust. It’s nothing to be ashamed of and they will want to be there for you. My family and friends were brilliant even when they didn’t know what to say.

Stay off the internet. Stay off the internet. Stay off the internet. Instead, buy Brian Dillon’s Tormented Hope: Nine Hypochondriac Lives. The introduction is like a balm for troubled nerves.

Order a copy of The Stranger by Kate Riordan (Michael Joseph, £12.99) for £11.04 at guardianbookshop.com