I agree with the “don’t tell about the cheating” sentiment, with the caveat that if there’s a real risk of him having been exposed to STDs, then you need to tell him.

If you don’t think that’s likely, rip the band-aid off quickly and permanently if you don’t want to be with him.

I will add: On the odd chance you love him and want to be with him, but are punishing yourself for cheating on him, there’s potential to move on from this with enough honesty. But this doesn’t seem to be the case here.

What Kate and Anonymousse said.

Look, there’s likely a reason your cheating. There’s probably something missing from your relationship and you seeked it elsewhere. For me, it was chemistry. I never wanted to have sex with my ex-fiance. For a while I thought it was fine. It wasn’t. It wasn’t until we were engaged that I realized this couldn’t go on forever.

Do yourselves both a favor and call it off. Getting married would be a huge mistake. He deserves better.

Honestly, you can keep seeing this other guy if you want, but I think it’s a bandaid and won’t last. You’d be better cooling off for a bit and doing some self reflection of why you let this happen. Either on your own but a therapist would be more helpful.