I count myself lucky – I am in my 30s and my parents are still together. My grandparents were married for many decades until my grandmother died suddenly. I have had fantastic role models when it comes to relationships. Then something happened that shocked me.

My grandfather was recently diagnosed with a fatal illness, and one day he decided to confess certain things to me. We have always been close, and I guess he felt he couldn’t burden anyone else with what was on his conscience. He told me he had been in love four times during his marriage, had been unfaithful and had another child I didn’t know about. This has sent my head spinning and made me question the sanctity of marriage. I know it sounds irrational, but I can’t help but feel sick at the thought of going to two weddings later this year.

I’m sorry your grandfather is so ill; on top of this, the news of his past must have come as a shock. We often have fixed ideas of what older family members are like, and tend to define them by the roles they have in relation to us. If we get on with them, our personal narrative is often to do with feeling comforted by that familiarity, and we don’t like it challenged by new information. So, to you, your grandfather was a grandad first, and a man second – that’s normal. But what your grandfather has done is destroy that image you had of him.

I talked to Nicola McCarry, a family psychotherapist. She thought it was wonderful that your parents and grandparents provided such good relationship role models, but she wanted you to think about why that was. Do you equate the number of years someone has been in a relationship with quality? Or is it also about something else?

What you observed between these couples that made you think they had “fantastic” relationships was probably real. You don’t know what went on with your grandparents. “Affairs can cause such heartbreak, but your grandmother might have known and chosen to stay married,” McCarry said.

I’m not sure if your grandfather will still be alive when this is published, but if he is, could you talk to him about it further? He has obviously decided to change the relationship between the two of you. This is uncomfortable for you, but maybe you could be curious about his life and why this happened? I wonder why your grandfather told you, and what he wanted you to do?

Of course, there’s also the other child he fathered, and I’m not sure what, if anything, you want to do about that. McCarry wondered if there was anyone else in the family you could talk to about this?

What perplexed me is how strongly you have reacted, in terms of what it means for the weddings you will go to this year, as if your grandfather’s infidelity has tarnished all marriages. It hasn’t. Do you have any idea why you might feel this way? What have your own romantic and sexual relationships been like? I wondered if perhaps you had this halcyon view of partnerships based on what you thought you saw between your grandparents and parents? As a consequence, have you “held out” for the perfect relationship?

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“Long-term relationships need a lot of work,” McCarry said. “And people can be faithful but still be very unkind to one another; people can be unfaithful but very kind to one another.”

Is it just the infidelity, or that it chimes with something else? If you saw your grandparents happy and in love, maybe they were, despite everything? McCarry suggested: “You can decide that everything you knew about your grandfather turns to ashes. Or you can think to yourself that people make mistakes and that’s just one part of your grandad. Because your grandfather did what he did, it doesn’t mean the loving relationship with your grandmother and with you wasn’t real.”

Realising that good people are flawed, make mistakes and still make a success of their life (because it sounds as if your grandfather did, given how fond you are of him) is a really valuable lesson to learn – about them and about yourself.

• Send your problem to annalisa.barbieri@mac.com. Annalisa regrets she cannot enter into personal correspondence.

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