Thanks, Dadaist - and you have got it right again - the deceit does compound the whole into something quite shockingly painful. Yes, it's not just the fact that the body that you thought was yours is discovered to be territory that has been pretty much explored and trampled over by someone else, but also that the person you had always hoped and trusted to be ultimately on your side had literally been in a corner with someone else, and not your corner.
I think that whatever the relationship is, and however strong, there is always that little bit of you somewhere that just trusts to luck that the other person really does completely want you, and therefore wouldn't do anything to lose you. There's that bit of yourself you can never quite see - your blind spot is truly to know the effect you have on another, how you're seen, how you're wanted. That's the bit that perhaps is our humility, our vulnerability, the bit of us that as teenagers made us sometimes just want to hide in a dark room at parties. The very centre of us, that we can only trust is seen, and wanted.
I guess that may be the bit that Gilesetc. was trying to sear into confidence with his fling, but, of course, it didn't work. After a betrayal, it's easy enough to find other people to validate your sexual appeal. If you put your mind to it, you can get validation from eyes in the street, glances in coffee shops, noticing the tone of voice and the slight tremor in the hand of the shop assistant. It isn't hard, if you tune in to it, to check your effect and appeal. But although that's reassuring, it's not really what you need. After something as devastating as betrayal by your bedfellow, lover, parent of your child, your physical home ... God, what do you need? I don't know yet. But it isn't someone else saying they want to fuck me. That's nice of them, but I need something else.
Actually, you need truthful emotion. You need someone who hasn't lied to you. You need someone new, and more decent. You need someone who can be entrusted with your heart.
Yeah, that must be it - that tiny bit that always feels it has to trust it's cherished - that's your heart.
Can you cheat and love? Yes, but you're not very good at loving if you do.