We have an elaborate way of getting the Christmas tree home this year, all 6ft of it. The lighter end is tied to my four-year-old son Tom’s scooter.

He pushes the scooter while I carry the heavier end in one hand and push Oliver, my 14-month-old son, in the buggy with the other. It’s not an easy task.

“You’ll be okay?” Eddie, the Christmas-tree man, asks, and Tom laughs his deep belly laugh, because he loves the chaos we’re causing in the high street. “My mum’s much stronger than she looks,” he says, which is what I’m always telling him.

The light is fading and the school pickup is going on around us as we head home. A jam of BMWs and 4WDs clogs the narrow road that runs alongside the common. We stop to rest by the first bench, because my arms are aching. That is when the shiny, new Range Rover stops a few yards from us. I glance at the driver. The person I most dread seeing is behind the wheel.

There she sits, in the familiar pose, straight-backed, staring ahead, immaculately dressed as ever, hair pulled back from her pretty face. When I look down, my hands are shaking. The pain is as unbearable as it was a year ago. I don’t know if she sees me, but if she does, she doesn’t wave or call out as she once would have done. I, too, keep my head down and pretend I haven’t seen her.

“Why does Santa have a beard, Mummy?”

Tom asks, and I look over at his young, wide-eyed face.

“Because it keeps his face warm in Lapland, darling,” I say. Tom thinks about this.

“Maybe he doesn’t have a razor in Lapland,” he says eventually.

“Maybe.” The lorry has driven up onto the curb, and the Range Rover moves on. I catch a final glimpse of the woman inside. Her name is Rebecca. She was once my best friend; godmother to my elder son, as was I to her eldest daughter. We have known each other since school – 20 years of loving friendship. She and her husband had even moved in a few streets away from us.

“Come on, Mummy,” Tom says, and we struggle on homeward. By the time we get there, the sun has set. The house sits in darkness and I wish I’d left the lights on so that it seemed welcoming. I put on the stereo. “Don’t know why, there’s no sun up in the sky,” Ella Fitzgerald sings out.

Rebecca had not been happy the year before last. “I haven’t done anything with my life,” she kept telling me, which wasn’t true. She’d had three children, been a classic domestic goddess. She had been married for 10 years to a banker, a good family man – quieter, less sociable than she, but solid and kind.

They belonged in the area more than we did: they were part of the wealthy City set. We, in contrast, were bohemians: two writers, the sort of couple everyone wants to invite to their parties, but whose life nobody actually wants to live, because we never knew where the next pay cheque was coming from.

I had known she was unhappy. When, one day, she came to me crying, I told her to slow down, to find things that were meaningful to her. She said I was right. She would do that. But she never did. She still rushed around, spent too much money on things she didn’t want, invited to dinner people she didn’t want to see, filled her diary with dates she didn’t want to keep. Like all of us, she was trying to fill holes in her life. She was just trying to fill them in the wrong way.

Tom is standing with his hands on his hips, looking at the tree lying outside the front door. “I suppose it’s up to me to fix this, Mummy,” he says, and tries to lift it into the house by himself. We struggle to get the tree into its stand, him trying to help but only hindering, and me looking as if I’m not helping while almost breaking my back. Then Tom and I set about hanging baubles on it, while Oliver sets about taking them off.

Last year, when Rebecca’s youngest was 14 months old, something changed. She blossomed. There was a flush to her cheeks. She lost weight, dressed differently, looked sexier. I thought she was finally filling the holes.

What of me at this time? I had my own holes. I was pregnant with my second child and suffering from acute morning sickness, but more than that, I had a dark lethargy hanging over me. There was a sense that something wasn’t right, but because I couldn’t identify it, it manifested itself as confusion inside. I couldn’t make decisions about the smallest things.

When Oliver was born, I should have felt on top of the world – I had given birth naturally to a healthy baby. But by the time he was two months old, I was as low in self-esteem as I had ever been. It was then that I found out.

“I’m having an affair with Rebecca,” my husband told me. And a week later, he left. We had been together for 14 years. He was my first love; I was his. He had been my life, my home. I thought I was his. I can hardly remember those first few months. I staggered through them, racked with the pain of the past and the terror of the future. I hid my tears all day and let them fall at night, trying to catch them when I fed Oliver, so they wouldn’t land on his head. I lost 2st.

I watched Tom retreating into himself. “I wish it was the way it used to be when Daddy was at home,” he kept saying to me. I would take his tiny hand in mine, and I’d know he and Oliver were the most precious things in the world, but even they couldn’t dull the pain, because they were so intrinsically entwined with it. Somehow, I had lost them the chance to live in a normal family, safe and secure with two parents who loved them enough to work at staying together. For better, for worse. To me, family was sacred.

After betrayal, was there vengeance? In my case, no. I didn’t throw eggs at my friend’s Range Rover. I didn’t scream or shout or smack her in the face. I did nothing. I was still breast-feeding, still up at night with a newborn. I can understand the emotions that led her where they did. And I do believe there is a place for passion in life, a way to enrich the ordinariness of every day. We all have holes we are trying to fill. But surely there are better ways? Nothing can be worth this much pain to so many people.

Statistically, women are now committing adultery as often as men. Financially independent, they aren’t so afraid to venture out and get what they want. But what of the sisterhood? How can a woman, who should instinctively know the place another woman is in, still choose to take a path that will cause so much damage? Falling in love is the easiest way to fill a hole, to bask in the reflected glory of someone’s heightened vision of you. It doesn’t last. We are all heaven and hell. Shouldn’t we find other ways?

Until we do, we’ll just have more affairs, break up more families, commit children to a life of segregation, shifted from one house to another like potted plants. Married love versus new love. In my case, new love won. Sort of. A year on, nobody is happy. Rebecca and my husband are still together, but the relationship seems tenuous. They live separately. Whenever he and I see each other – every other weekend, when he has the children – we meet with eyes downcast and speak in monosyllables. What we once were has been broken. We are meant to be getting a divorce.

We have finished the tree, the boys and I. Most of the baubles are on the top half, where Oliver can’t reach them, but it looks lovely, and I’ve made a fire that is roaring in the wood-burner. We sit beside it, in the house I’m going to have to sell because I can no longer afford to live here, and roast marshmallows, just the three of us, as we will be this Christmas time. Slowly trying to fill up the huge space where someone is missing.

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