A woman still haunted by an argument from years ago wonders what to do. Mariella Frostrup says its high time to eat humble pie

The dilemma Many years ago, when my son was 18, he met a girl who had a young baby. When the child was three years old, I was asked to babysit. I was delighted and we had fun decorating my Christmas tree before I put the little boy to sleep in the double bed that my son and his girlfriend would be returning to.

I was woken by him crying for his mum so I lay on the bed while he fell asleep. I also fell asleep. Later I was woken up by my son and girlfriend and I got back into my own bed. The next day the girlfriend said she needed to speak with me and my first thought was: Oh dear, what has my son done?

I was absolutely devastated when she said: I need to know why you were in bed with my child? I know now that I acted immaturely, but I turned round and said: Get out of my house and never come back. I was so shocked at the insinuation. My son has never forgiven me and throws it up at every chance he gets, saying she was in the right, she was protecting her son and now it is really depressing me.

Mariella replies Many years ago? Im hoping thats a misprint. If its been haunting you that long no wonder its depressing you. This certainly isnt a scenario that should be lingering and leaking its poison into your life after so many years.

There are few among us who arent occasionally haunted by past mistakes. Only the truly bland or totally saintly exist without a scattering of shameful scenes long past that bob back into view at unpredictable moments. For most of us these are memories wed prefer to have abandoned in the second of their creation, not find ourselves being stalked by in perpetuity, ready to be relived again and again whenever they slide to the fore. They are the thoughts that seep into our unconscious in the dead of night and prove hard to banish, or come scudding across the horizon on a blue sky day, as unexpected as they are unwelcome.

What makes this tale of yours so frustrating is how easily it could have been dealt with all those years ago when youthful inexperience married to mature pride caused both of you to behave badly. All that was really called for was to diffuse the emotions riding high with a frank conversation, hopefully followed by a cup of tea or something stronger. Left undiscussed for decades, youve allowed this innocuous moment to gather potent power and create an ongoing rift that needs to be bridged.

Your behaviour while babysitting is not the issue. From what you describe your perfectly harmless attempt to reassure your charge was as understandable as it was normal. Anyone who has ever had children or even just babysat them knows that when they are young and upset there are few things more reassuring than the warmth and comfort of another body making them feel that all is well with the world.

Being right in this situation doesnt make you blameless. Your sons girlfriend must have been pretty naive and impulsive to have approached the issue so confrontationally. It would have been so much easier simply to have had a civilised conversation about how the night went rather than to launch an offensive missile. Shes definitely more of a North Korea than a Netherlands! Your response, I have to say, was a bit Kim Jong-un too. As youve already admitted, it was way out of proportion and unlikely to inspire anything other than anger and mistrust.

Nevertheless I am a little amazed that the repercussions of that skirmish continue to this day. If you are still chewing the fat so many years later, its definitely time you took action to clear the air.

Could this now simply be a symptom of a faulty line of communication thats developed between you and your son? An apology followed by a sensible conversation is long overdue and you are the one with most to gain by initiating it. If you cant entice them both to your house, then ambush them in theirs, admit your mistake in reacting as you did and be honest about how it continues to trouble you.

If youd taken proactive steps to appease them long ago, you could have brushed this off like fluff on a jacket, but having left it to fester the only choice you have is to claim the blame as a result of your over-reaction and try to normalise relations. You did nothing wrong the night you babysat, but youve been in the wrong since the day you threw her out. Generously accepting culpability now is the easiest way to move on.

Make amends for the sake of your own sanity and to improve your relationship with your son, eat a chunk of humble pie and free yourself up for bigger and more interesting thoughts and torments.

If you have a dilemma, send a brief email to mariella.frostrup@observer.co.uk. Follow her on Twitter @mariellaf1

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