It tore apart the life she knew and devastated the man she cared for most in the world. But there was just no stopping it, says Ruby More
Alone in our cottage I began the overwhelming task of dividing up what we used to share. Who would keep the Blue Aeroplane albums, the Indian wall-hanging, the stainless steel bowls? If I left him our most favourite things would his pain be less? Would my own guilt be relieved?

A and I were the couple our friends longed to be. His humour offset my seriousness, my clarity offset his impetuousness; we respected each other's need for both space and closeness. For 12 years, we did everything possible to cherish and protect each other from pain. Now I felt like a mother turning her back on her child; hurting myself by causing him pain - and worse, I wouldn't even be there to help him through it.

B had been an object of fantasy for a year. When I was in the same room as him I avoided eye-contact, then spent weeks fantasising accidental meetings. I didn't want trouble. Nevertheless, one night, out in a nightclub with a girlfriend, B's eyes met mine. It was astounding: red face, jelly knees, the rest of the room went out of focus.

The next time I saw B, I agreed to meet him for a coffee. I hoped he would turn out to be a moron - shallow, boring, self-obsessed, a snob. Any of those would have helped. But honest, vibrant, sharp-minded, funny... Over cappuccino in a dark-windowed coffee house, I told him: "I live with my boyfriend." "Then why are you here?" Why was I there? I felt I had no choice. It was bigger, stronger, more determined than I. I fell head over heels. I couldn't help it. I felt as if I'd found a part of myself that had been missing forever.

The hard part: telling you about the gorgeous, spirited, charming, tormented, brilliant man I left. A was the closest person to me in the world. He was - still is - a friend of my heart. We had weathered storms before, always come out holding hands, feeling closer. But not this time.

One night, A said he could see B made me happy; he didn't want to hold me back. If you truly love someone you want the best for them. We'd always believed that. Now A was putting our beliefs into action. But, to protect himself, he asked to be left alone in our cottage for a week. After that, we knew there was no going back.

Leaving A, I felt I was leaving a big chunk of me behind. We wept together, yelled, wept some more; felt like children trying to comfort each other. We did not know how we would survive apart. It was like losing a limb. But I had to be with B - B who made me feel more alive, more excited than I'd ever felt before.

I took the minimum of possessions and moved into B's flat. I thought things would continue as they started: flowers and brioche for breakfast, long lazy days spent talking, a love bubble for two. I hadn't bargained on how much I'd miss A - his familiarity, the way he made me feel safe and known. I hadn't bargained on how much I'd hurt - for me, for A, for all the things we had between us.

And, after 12 years with A, I hadn't bargained on how very different B would be. A used to ask, "How was your day?" B asked, "What are you going to do next?" A used to say, "You're great as you are." B said, "You can be better." The things that had so drawn me to B now began to make me feel like a stranger in a foreign land. I didn't know his rules, language, behaviour. And, after 12 years of A's concern and interest, I found it impossible to break the habit of confiding in him. Understandably, this made B angry, insecure, jealous.

One night, I told B I had made plans to go to Manchester with a group of people. He went berserk. He told me: "You're not going." Not going - I thought he was joking. When I realised he was serious, I was outraged. I'd grown too used to a man who was happy for me to come and go as I pleased. I was not accustomed to orders. There was only one thing for it. I told B it was over. "Get out," I screamed. "I hate you." But as he tried to leave, I found I was wedged between him and the door and begging: "Don't leave me, please don't go."

I used to have a list of won't-tolerates. When friends talked of their torrid tangles, I would say: "Never let a man tell you what to do or make you feel small; don't put up with domination..." Now, I was turning into the woman I always said I would never be.

B and I fought like cats, hissing, spitting, arching backs. This would have been bad enough; two weeks of caterwauling and you'd leave, right? The worst of it was, we still burned for each other, ached. Was it the fact that I still loved A that made B so impossibly insecure and controlling - or was it his personality? The see-saw began. I moved out of B's flat, back in, out, in. I ripped up telephone directories, covered his kitchen wall with hurtled fruit. Love-sick. When it comes to love, some of us are like greedy children in a sweet shop: vomiting only to make room for more.

Six months later, I got my own place. Maybe this was what it was about: learning to live on my own, aged 33. At first, B was around too much. Then, suddenly, he wasn't around enough and I felt empty. I was not as resourceful as I thought I would be. I didn't go for long walks, read novels, write/ paint/draw. I watched too much TV, had lots of baths, cried in bed, talking interminably on the phone to friends, who said: "The tide always turns at the lowest point."

What the tide actually does is rise then crash. Eighteen months since the fateful cappuccino, B and I are still trying to find a way to ride the waves rather than be tossed about by them. We get along great for a week or two, then a message from A on my answering machine opens up old wounds. Although I have chosen B, I cannot cut A out of my life. It is an impossible situation. Life isn't easy or cosy. It's exhausting and exhilarating - I feel alternately desperate and delighted. B stays, leaves, stays, leaves. Late-night phone calls are like injections of pain. They are addictive, we keep going back for more.

A and I meet every few weeks. Time together feels relaxed and precious, but at the point of parting I feel my insides leaving with him. Our friendship is a very deep, unbreakable, almost umbilical love. Sometimes, I think A and I were more like brother and sister than lovers. Other times, all there are are memories of A: places we've been, things we've done, his face, his voice, over and over. I miss the way we were together, our lifestyle, our home, spending mutual time with mutual friends. One hundred and one little things that long-term couples take for granted - the threads of the fabric of the relationship, the things which are so cosy, familiar and comfortable, they are easily overlooked.

I am desperate for A to be happy. His pain feels like my pain. His loneliness is almost too much to bear. Each time he has a new girlfriend, I feel relieved. Each time they break up, I feel relieved. I want him to be happy but I don't want him to replace me. On bad days, I blame myself for throwing away the oasis in my life. But we are good friends. We always were. Some things are built to last.

I wanted passion and joy. For that, I sacrificed calm, familiarity, harmony. I swapped a sand dune for a roller coaster, a swimming pool for the sea. B and I cannot live together, but we cannot live apart. We do not gel, but we cannot seem to un-gel. Love is deepening between us, but it is a slow, cautious, mistrustful thing.

I may have bitten off more than I can chew, but that doesn't mean I don't recommend it. Some days are like walking on marshmallows. Very often, when B and I are kissing and getting into each other, it feels like the first time. I feel more alive, more free, more (wonderfully and horrifically) me than ever before. I left A for B, looking for a happy ending. But maybe there are no happy endings, just different ones.