I wrote this on the Paul Hollywood thread but feel it has some relevance here -
OP, you are going to be judged horribly on this thread because we seem to be stuck in a Victorian hangover of morality in this country. We love black and white tales where the other woman is the “harlot”, the partner is the “wronged person who can do no wrong”, etc. It’s why so many Daily Mail stories about celebrity separations get tons of hits.
But the reality is, it takes a very certain and clear individual to walk away from some relationships. Not everyone goes “I am not fulfilled here, so I will walk into singledom with confidence”. Often we don’t realise that we don’t truly want our current partner until we meet someone else. This is not me advocating immorality; I’m stating plain fact.
I think there’s more going on than whether you should tell him. You claim “he’d have you back in an instant” ; is there a power dynamic here which means you feel contempt for him, and his sympathetic understanding of you, enough to cheat? Does he fill your emotional landscape, or is he a “rock” for you to hang your life on, yet you remain otherwise unfulfilled?
Telling him gives him power which is important here. Cheated people deserve self determination. But if he says he won’t leave you, that’s not the end of it.
Ask yourself what cheating filled in your life (no not that, I mean emotionally).
Ask yourself whether the risk of unprotected sex was part of it. The thrill of closeness and reckless abandon with another. The “I don’t care about anything or anyone” feeling.
There’s so much going on here, but all I can say is I wish you well, good luck with everything, and hope you can find your emotional path. 