There is no adequate way to describe the shock and fear of a cancer diagnosis. When I got a letter after a routine mammogram asking me to present for further tests, I had a very bad feeling indeed. Even though I was assured that nine out of 10 women who were asked to return had no "breast disease" (a coy euphemism, I thought), I knew I was the 10th.

At the hospital a few days later, I was shown into a small room containing a couch and a doctor. Bidding me lie down, she pointed to my mammogram which was on display like some perverse piece of art. There was a clear, white area standing out from the cloudy grey mass of the breast tissue. "That's why we wanted you back," she said in a declamatory sort of way. "I need to do a biopsy."

Within a few days I had a lumpectomy, followed by months of chemo-therapy and then a bilateral mastectomy. The chemotherapy was hell on earth, but the surgery was a doddle. I was very strong and positive. I carried on working throughout, doing a job that involved long hours and constant travelling. I was Superwoman in a wig.

My upbeat attitude at that point came from several sources. I had a good prognosis. I was doing a job I loved. And as the author of several books about women, weight and prejudice I was known for being strong, campaigning and someone who did not take no for an answer. I had fought prejudice; I could fight cancer. It takes immense willpower to survive and succeed in a world that tells you that you are no good if you're fat, so I used that willpower in the way I handled having cancer.

I was also seduced by the cancer myths. These are the messages that come from books, magazines, TV programmes, friends and even from the news. They are a subtly prescriptive, quasi-spiritual bunch of exhortations and examples which, if followed, suggest the best chance of survival. They all come under the one big heading of "positive thinking".

Around that time I joined a cancer support group and discovered another, very distressing cancer myth. This is that families are brought closer by the illness; that they all pull together, united in support and love for the sufferer, and that the experience adds a special dimension to family relationships. Listening to the pain of some of the group members, I found that this is not always the case.

One man, John, cried week after week as he told how his parents had closed ranks when his father's bowel cancer was diagnosed. It was something they felt they wanted to face as a couple, and although they had previously been a very close family they did not want John to be a part of what they were going through. His attempts to get information about his father's condition were met with minimal response, and his offers to find out about new drug therapy and other treatment were regarded as interference. When his father had finished treatment and was in remission, John felt he was readmitted, but he now feels that things will never be the same and that nothing positive has come out of the experience.

A woman with endometrial cancer felt she had lost her adult daughter, who had become distant and apparently uncaring. Their relationship had previously been very close, and this woman could not understand why her daughter had abandoned her. The counsellor in the group explained that cancer can seem more frightening to the family than to the sufferer, and that some people can cope with their loved one's mortality only by burying their head in the sand.

Another woman, with secondary breast cancer, talked about how much she needed her husband to accept that she was going to die. His absolute refusal to talk about it had driven a huge wedge between them. Ironically, she had cared for him for several years - he had Parkinson's - and it was always assumed he would go first. He simply could not face up to the twist that fate had delivered.

James Brennan, a clinical psychologist at the Bristol oncology centre, says difficulties in family relationships can be a big feature in the cancer picture. "Sometimes those you most expect to be there just disappear. Very often it is women who are disappointed at the level of support they receive, almost as though people cannot accept them not being in their usual slot."

Over the weeks I heard that most of these rifts were healed, and that very often families became closer as a result. It is hard to deal with, though, when family members react in ways that belie the myth of instant unity that our culture has created.

I chose to think of cancer as a chance to reinvent my life, to strengthen relationships, mend fences and to make the most of every day and every opportunity. I kept reading about cancer survivors who said that every moment was precious and that they were never troubled by the little things that used to irritate them. Nothing got them down any more; everything was "a challenge". I made a firm resolve to take up regular voluntary work. I would be nice to those people who irritated the hell out of me. I would not watch daytime TV. And never, ever again would I complain about life's little niggles, because what were they, compared with being free of cancer?

Cancer myths are like a religion. There are rules. In order to be saved, you must change. In fact, you are responsible for your cancer, you need to find out why you have it. It is a metaphor, a signal, a wake-up call. So you must cleanse yourself mentally, emotionally, physically, spiritually.

It also helps if you run the occasional marathon or climb a mountain. And if you wake up to grey skies and pouring rain - well, rejoice in the freshness, the renewal of all green things. On no account put your head under the bedclothes and say "bugger it". If your internet connection goes down when you have work deadlines, do not get frustrated or irritated. That will cause stress - and, as we all know, stress causes cancer.

My initial positive attitude fell apart when my job and my treatment ended at around the same time, and I fell into a deep pit of depression, exhaustion, and negative thinking. I was haunted by the fear that the cancer would return and I found it difficult to leap out of bed and smell the roses. And because the cancer myths had led me to believe that my state of mind was of paramount importance to my survival, I feared I might be the architect of my own destruction, that I could save myself if only I could cultivate the right attitude.

I also felt enormously guilty. People with a far worse prognosis than mine were doing wonderful things in their lives and seemed to experience a joy that was eluding me. More guilt. Not only was I not doing something positive to show my gratitude, but I wasn't feeling the way I should. I told my surgeon how I felt. "You must think I'm an awful whinger," I said. "You're dealing with people far worse than I am. I've been lucky."

He put things into perspective for me. "You're not lucky - you've had cancer!" he said, and reassured me that my feelings were absolutely, blessedly normal.

James Brennan feels strongly about the tyranny of the cancer myths. He believes that cancer patients have enough to deal with without the pressure of having to think positively. "There's a huge level of personal responsibility for illness put on people," he says. "This is an oppressive idea. We get pretty fed up with it here."

I'm not surprised. Only this morning I read in a book about cancer prevention (written by a doctor in the vanguard of cancer treatment) that cancer sufferers die sooner if they are depressed. And "it is very likely that prolonged stress or severe shock and upset can allow abnormal cancer cells (which would normally be spotted and destroyed) to escape surveillance and survive."

This book also informed me that people who "practise the mind-body technique of visualisation whilst having chemotherapy have better survival rates". Great. I didn't do that. Coming from an ethos that embodies the positive thinking model, that seems to me to deliver a pretty negative message.

Brennan also has hard scientific facts about the stress myth. "This is dangerous territory," he says. "There have been hundreds of epidemiological studies but not one shred of evidence that stress causes cancer."

That is the best news yet to emerge from the cancer myths quagmire. Believe me, you can be sucked into it because you will do anything to prevent the recurrence of cancer.

I have made life changes. I've become enjoyably reckless. We've bought a house in Italy that we can't really afford. And my husband is leaving work and taking the risk of going freelance so we can have more time together. I don't care about pensions, an attitude that seems to shock people!

And I've come to accept my own nature. I love life, but I still have bouts of depression and anxiety. The little things do get me down sometimes, and I'm sure there are plenty of people ready to confirm that I'm still not dispensing peace and love. I did try, but then I became my cantankerous old self again. Besides, I would hate to think that cancer survival requires a saintliness that would render me ineligible for membership of the mordantly witty "grumpy old women" fellowship whose television series makes me laugh so much. I'm afraid I'm not going to join those admirable cancer survivors who run marathons or climb mountains for charity, though I will try to do something to help others. Not until I've got myself sorted out though.

And cynical though I might sound, my life is richer, mainly because of the people who are seeing me through this. You know who you are.

· Shelley Bovey is the author of What Have You Got To Lose? The Great Weight Debate, published by The Women's Press.