Focus on your marriage first – and deal with it kindly. Then decide what to do next. That’s the grown-up thing to do

The dilemma I’m in love with my wife’s close friend. These feelings built up over a few years, and when we were drunk one night, I told her. She said she felt the same. Later we met and chatted, but the net result is, because of her friendship with my wife, nothing could happen. She is now in another relationship and has moved on. I understand I’m the bad guy here, but it hurts that it’s circumstance rather than her not being interested in me.

I’m struggling. I’ve fallen totally in love and think about her every waking minute. I think she represents everything that is missing in my current marriage – the spark for me has gone. I love my wife and care about her, but ultimately don’t fancy her any more. Is this normal for a marriage of 10 years? Should I be content with a woman who loves me, or does it mean I have some sort of mental issue for not being able to move on and enjoy life?

It’s starting to make me sick and depressed and I desperately need to stop thinking about her. I think of her having sex with other people and it tears me apart. The thought of breaking my wife’s heart is horrible, but I don’t want to spend the rest of my life thinking what could have been.

Mariella replies Well, that’s up to you. Whether or not you were to hook up with this current object of your desire, I have a feeling you are still capable of spending the rest of your life in regret. Not necessarily for missing this particular opportunity, but because you’re displaying classic symptoms of “the grass is greener” syndrome – and that tends to be a recurring condition.

You ask if it’s normal after 10 years of marriage to be feeling this way and the good news is that you are truly unexceptional. There are very few couples who, after more than a decade of co-habitation, are still powered primarily by desire. Instead, what develops in the best-case scenario is a hybrid where love, respect, reliance, trust, friendship and mutual interests, plus enough sex to keep it from being an omission, keep you going. Certainly, some couples keep the flame doing more than a gentle flicker, but they are the exception rather than the rule. In most mature relationships, sex is like sleep: something you need, but often don’t get enough of.

She is not relevant to your situation or your ticket out of it

I’m not suggesting you throw in the towel and resign yourself to a passionless life for the sake of your wife’s feelings, but in all relationships the scales eventually shift from desire to connections beyond that primal instinct. It does mean you have to compromise and the question today is how many of us are prepared for that in a world that keeps telling us we should be having it all?

If you feel the relationship has run its course, you have every right to call it a day, but don’t confuse the possibility of moving on with the ambition of seducing your wife’s friend. Saying you are in love with this woman and can’t escape painful fantasies of her sexual activities just confirms to me that this is not a love affair. You’re simply dumping your unrequited desire on to the nearest set of shoulders. It’s hardly a boldly imaginative act and would be the ultimate betrayal of your wife, as I hope you are aware.

I’m not saying this woman hasn’t sent you mixed messages. While I applaud her decision not to go further, thanks to her relationship with your wife, I question the wisdom of her admission that she, too, shares feelings for you. I wonder if she was just being kind. Please don’t take that the wrong way, but when I imagine myself in such a situation, with my mate’s husband confessing his lustful desires on a drunken night, I suspect I’d do much the same as her. I’d try to defuse the situation kindly, while firmly calling a halt.

She is neither relevant to your situation or your ticket out of it. She is what she’s always been, which is not your potential girlfriend but your wife’s pal. There’s one thing I know for sure after my long years in this job and that is that there are plenty more fish in the sea. Ending your marriage before you locate a substitute is by far the superior way to solve your apparent woes.

But remember, as I outlined before, that no partnership can survive the expectation that early levels of passion will remain forever. Our relationships are subject to the vagaries of fate and the wear and tear of daily living. Early passion will always segue into something less potent, but possibly more rewarding.

You describe your relationship as a happy one and that is something you should be careful not to undervalue. I recommend you think seriously and carefully about the wisdom and potential fallout of taking further steps in an alternative direction. Instead, maybe it’s time to summon up some moral fibre. If your current love life is unsustainable, close that chapter kindly and emphatically before stepping beyond it into a freed-up future. That’s certainly the grown-up thing to do.

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

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• This article was amended on 3 March 2020 to remove some details.