Sex File: I can't stop fantasising about my colleague

You appear to have a solid marriage, but couples who are happily married don't as a rule develop raging crushes on their colleagues. It is worth considering whether what you are feeling reflects any conscious or unconscious dissatisfaction with your relationship. You have done nothing wrong, but secrecy can feed sexual desire and be a big driver for infidelity. All it takes is one too many after-work drinks and this situation could rapidly spiral into one that is much more difficult to unwind. Affairs are an addictive but predictable adrenaline rush of sneaking text messages, meeting in hotel rooms and trying not to get caught. The intoxicating mix of sex, guilt and shame has kept many affairs going far longer than if they had been in the open.

That you feel guilty suggests that you already recognise you are crossing a line, so I would urge you to stick a pin in this balloon as soon as possible. One way to reset the dial would be to — gently — tell your husband how you are feeling. That probably seems like a terrifying and disproportionate response to a harmless crush, but it is the quickest and most effective way to break the spell. Your husband will probably be shocked and hurt but rocking the boat a little can jolt a relationship in a good way.

You may have been married for 17 years, but you are autonomous individuals and either of you could choose to leave if you wanted to. That realisation can be just what is needed to stop couples from taking each other for granted. Your husband's reaction will also help you to evaluate your feelings for your colleague. When you are nursing a secret fantasy you don't take reality into account, but once you are aware of the damage that emotional or physical betrayal would do to the man you love, things get real.

If you cannot face telling your husband, I would urge you to try to make your sexual fantasy about him instead. You get to write the script so think of crazy things you would like to do with him in the most exotic places.

In 2018 the Israeli psychologist Gurit Birnbaum asked study participants to fantasise either about a partner or about someone else, and then rate their desire to engage in sex and other activities that were beneficial to their relationship with their partner. The study showed that when they fantasised about their partner, as opposed to someone else, it not only heightened desire, it also enhanced their relationship and made their partner more appealing.

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