I refuse to be the stereotypical bitter single mum. Her age makes it easier. She has middle-aged spread and a lived-in face and he will probably end up caring for her in a few years

Perhaps I should find it harder knowing that my husband would prefer to be with a woman more than 10 years older than me. A woman who is not far off her 60th birthday. A woman still reaching for the henna hair dye despite her advancing years.

She is old enough to be my daughters’ grandmother, never mind potential stepmother. How insulting, right? And what an outrage! I’m younger, a toned size 10 and I look after my appearance. The humiliation should be devastating.

But, perhaps surprisingly, it makes things a whole lot easier.

There is absolutely nothing for me to be jealous about. No stereotypical younger woman with a pre-baby body and not a grey hair in sight. My husband’s mistress has middle-aged spread and a lived-in face. When friends first spotted them together, they reassured me that he must be telling the truth when he said nothing was happening between them. There was no way they could be romantically together as she was “so old”. How wrong we all were.

He still denies an affair even now, despite the overwhelming evidence to the contrary, claiming they formed a relationship after we had split up. But the signs of an affair were there long before the sickening suspicion and then, finally, the confirmation.

I can pretty much pinpoint when it started. From being my husband’s everything, it was as if a switch had flicked off overnight. Cold and distant, he took up golf and disappeared for hours at a time. His phone was permanently clamped in his hand, and he would need to make private “work calls” at weekends and when we were on family trips. All affection was withdrawn and his hair-trigger temper became apocalyptic as he clearly resented every second he spent in my company.

With hindsight, it doesn’t take a psychologist to work it out. He felt trapped in our marriage: we had two preschool-age daughters and he wanted his carefree life back. His mistress’s children are grown up, so she and he are free of responsibility or restrictions. A holiday touring around south-east Asia? No problem. A music festival in New Orleans? Let’s book it. Midlife crisis complete – he has even started dressing like he did 25 years ago.

I don’t blame his mistress one bit. She must have thought it was her lucky day when a handsome, younger man showed an interest. Maybe she thought she was destined for a life alone, or to be stuck with men of her own generation – with prostate problems and a cosy pair of slippers.

If it hadn’t been her, it would have been someone else. It is not as if he met the love of his life and had to be true to himself. She was just an escape route out of a life he viewed as mundane and humdrum until he didn’t have it any more and realised the grass isn’t always greener. Of course, life with two small children is hard – throw in a long daily commute and it is downright tough. But you deal with it and know that, for a short time, you might have to come a bit further down the priority list. Instead of which, he threw it all away for a woman he will probably end up caring for in a few years.

There were weeks of him sobbing and begging to come back, calling it the biggest mistake of his life but, by then, I had begun to experience how life could be, should be – fun, light-hearted and not living in fear of someone else’s mood swings. The cloud of doom had left the building and I was not going to let it back in.

Now things have calmed down and we are a few years down the line, I am glad he is with an older woman. He and I aren’t right together, and my daughters seem to like her. Because she is a mum herself, I trust her with my children and am happy there is someone else looking out for them when they visit their dad. Better they are staying in her beautiful home than a depressing bedsit.

Granted, this wasn’t the life I had imagined. The Richard Curtis world of happy ever after with a mum and a dad in a rambling house hosting big parties filled with children running in and out. We had talked about moving out to the countryside one day – dreams that were all whipped away pretty much overnight, leaving a void of uncertainty. But one thing I know is how unhappy the girls and I would be if their dad and I still shared a home.

Yes, things such as parents’ evenings, sports days and school shows can be hard when you are surrounded by other parents with their partners. Or when one of the girls has done something particularly funny or clever and you long to be able to exchange that proud look with someone who loves them just as much as you.

But the reality is, even if we were still together, those situations would not happen like that. He would be scowling and surly at parents’ evening, or he would refuse to talk or make eye contact with me at sports day. It would not have been the “normal” interaction I see with other couples. And, anyway, the older I get, the more I realise that quite often the happy facade many couples present is very different from the reality when the front door is closed.

I refuse to be the stereotypical bitter single mum: I am a professional fortysomething mother with a very busy, joy-filled life who just happens to be parenting alone. I don’t sit around swigging chardonnay and slagging off men. I love men – I have three brothers and lots of male friends. One bad marriage doesn’t mean it’s game over. Perhaps surprisingly, I don’t regret my choice of husband. We were deeply in love once and shared many special times. We also created two perfect little people. One day, I hope that I will find love again, but perhaps this time I will choose someone who has put their midlife crisis far behind them.