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.