APART from extreme physical pain or bereavement, few things can devastate a child more than the break up of a family. It was 25 years ago, when I was 13, that my two sisters and I listened to my father read a letter from my mother - saying that she was leaving him. I can still recall the sickening jolt, as our safe ordinary world slid into unreality.

We were a very ordinary family. We lived in a small Sussex town where both my parents worked. We weren't either rich or poor. And maybe that was part of the shock. At that time few middle-class marriages ever went wrong in public. I didn't know anyone who was divorced. I had, in retrospect, guessed at tensions between my parents - rows that cut off when I walked into the room; an atmosphere that had stopped most family events being fun. But nothing prepared me for the ugly shock of that letter.

My mother had gone away to friends - actually to stay with the man she'd met several months ago and fallen deeply in love with. When she wrote to my father she thought he'd stopped loving her years ago and that he'd be glad to be free. In fact he spared no one his bewilderment, hurt and rage. When my mother returned to collect some clothes and take us children to her lover's house there were terrible arguments, whose details I've edited out of my head. I can remember only the terror of not knowing what would happen next, of wondering how we would get through the day. Also the realisation that as the oldest child, I'd been foisted with a horrible new adultness. In front of me were two suffering grown-ups, not my Mum and Dad. They were too damaged themselves to be able to look after me and my sisters. I felt as if I had to watch out for the whole family as much as for myself.

I don't remember the details of how we left my father and went to my stepfather-to-be's house but I do remember the strange relief of it. Trying to make some kind of normal life out of the situation was odd and awkward, and not without pain. But it was the adults' responsibility to make things work and I felt a kind of weak, relieved passivity. The arrangement only lasted a few weeks though. Something happened between my parents to make them decide to try to mend their marriage, so we all moved back home with my father.

I'm sure they both made an effort, I'm sure there must have been good days during the year that my parents tried to stick their marriage out. But my only memory is of a war zone, of a home that had become a battleground. Every day I would get off the bus from school and feel as if I was stepping out of a secure, normal and blissfully boring life into a nightmare of claustrophobic tension and uncertainty. Was there going to be a row, and if so would it be safely over before supper, before I had to get my homework done? While we ate, would we manage to get through some safe topics of conversation? Would the arguing start again once my sisters and I were in bed? (It got so that I couldn't sleep until I was sure my parents were in their separate rooms.) Every day I felt like a tightrope walker.

I fretted helplessly, luridly about my mother's ability to survive the strain of home and the grief of giving up her lover. I'd arrive back from school terrified that she would have killed herself or run away. I found it hard to speak to my father, who I started to blame for everything. I didn't like to bring anyone home with me, in fact I lied to all but my closest friends about what was going on.

When my mother finally took her courage and left for the second time, the relief, if messy, was enormous.

Trying to construct a new family with my stepfather was weird but it felt as if things were moving forward. There were problems and conflicts of loyalty but they didn't paralyse ordinary life. I didn't think about them when I was away from home. I felt as if I could breathe again. The world felt solid.

It took a few years for my father to re-marry and I felt guilty about leaving him and being happy away from him. But we are now very close. He finally met another woman and my parents now see each other very amicably at big family events.

My parents' divorce turned out happier than many, but it is still grounds for questioning the belief that the children do better if their parents stay together. During the months that my mother and father tried to save their marriage my sisters and I became evasive, repressed and damagingly self-conscious. I'm convinced that if they had not divorced I would have left them as soon as I safely (or unsafely) could.

I can also mark up a surprising number of benefits that have come out of the final situation. Being forced to acknowledge and understand my parents' adult emotions meant that I was a less blindly bolshie adolescent than I might have been. And for years I've taken it for granted that I can talk with my mother and father, openly, as equals. I have had a pretty exhaustive course in how marriages work and don't work. Plus I have acquired a huge extended family of which I'm deeply fond.

It's harder to calculate the damage. Of course there are bad memories, possibly worse. But as the world goes, my sisters and I are lucky and successful. We are all established in professional careers, we all married young, and happily. I can't, right now, imagine my own marriage ever deteriorating. But if it did, I would never stay with my husband just for the sake of my children.