I can't believe it, but my father has been having affairs...

'Your father being a serial adulterer doesn’t make him a monster': Mariella Frostrup advises a distraught daughter. Photograph: PA

The dilemma Last night my mum called to say she and my dad won't be coming to stay at the weekend, as they are having a few problems.? She eventually explained that my dad has been having an affair.? Not just that, but he has been having affairs on and off for 30 years, but is impotent with my mum.?I feel unbelievably betrayed by my father. I have spent my whole life thinking he was this amazing man who worked hard, often away from home, to provide for his family, only to find out he has been hurting my mum. It has shocked me to the core. I always thought my parents' marriage was perfect – they seemed so much more in love than any of my friends' parents. I have based all of my feelings about relationships and marriage on this. I feel like I don't know who either of my parents are. I am 30 years old and have a wonderful and supportive husband and a fantastic life, but I now feel the rug has been pulled from under my feet.

Mariella replies What we presume to be the facts and what is real are often two completely separate matters. The veracity of events as they appear to us becomes what we believe to be the accurate picture. It's why history is open to interpretation and why science and maths are so addictive to their practitioners as the two areas of?knowledge in which you can actually repeat a scenario ad nauseam and come up with the same result. That's why I think fiction is the best place to examine the secrets of reality.

We homo sapiens are complicated, secretive, unpredictable, illogical and endlessly surprising – elements that have kept us on our toes and ruling the planet for quite a few millennia.

You haven't been betrayed; your parents are still your parents – you've just had your version of them fleshed out a bit. They are the same people, but they may have put your needs before their own throughout their married lives. I can understand you feeling let down by your father. You won't be the first little girl to bestow God-like status on Dad, only to discover that he was a mere mortal.

Your father being a serial adulterer may dampen your ardour, but it doesn't make him a monster. Neither should it follow that your parents weren't in love, just that their relationship was more complex than you perceived it to be. It may even turn out that they chose to stay together despite sexual problems because they loved each other so much.

Adults who take on the onerous task of raising children have the responsibility of protecting them from the full brunt of the grown-up world. Sadly, far too many of us fail, and many youngsters grow up in the mysterious shadowland of adult dysfunction, neither understanding nor emotionally processing what's occurring around them. If you'd known what was happening between your parents your security would have been shattered and you'd likely have echoed their fractured adult behaviour in your own relationships. I can certainly understand why this has rocked you to the core, but if you consider the alternatives, you may come around to their way of thinking.

The majority of parents find it a?challenge too far to keep the picture rosy, so it's to the credit of yours that they've managed to spin out the public face of happily married monogamy for so long. Would you swop the 30 secure years you've had and the model of a loving and supportive marriage it gave you for the discord and unhappiness that many of your friends will have witnessed in childhood? What you have been living with is not a lie as such but one dimension of a three-dimensional relationship. I admit I'm surprised it's taken this long for the secret to come out. Your mum may have sacrificed way too much of her adult life in the hope of change from her partner. She won't be the first or the last to make that mistake.

Again, you've no idea what secret deals were forged behind those closed bedroom doors. Uncovering the subterranean depths beneath people's perfect facades is rarely without shocks and surprises, particularly among those you assume you know, whether partners, parents or children. In the heat of such revelations it's perfectly normal to get things out of proportion and to feel the ground beneath your feet dissolve. Time, understanding, frank discussion and a big dose of empathy will help see you all through. What you've learned is that no relationship is perfect – and that may prove a?valuable lesson.

If you have a dilemma, send a brief email to mariella.frostrup@observer.co.uk. To have your say on this week's column, go to guardian.co.uk/dearmariella. Follow Mariella on Twitter @mariellaf1

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