Just ranting

I just found out my fiancée of 9 years has been cheating when I confronted him he lied saying it was for us, I love this man and I know if I stay he will do it again but how can I just move on this is who I wanted to marry, start a life with, have kids. I can’t stop thinking about him having sex with one of those girls, I can’t stop crying.

Background story: we are high school sweethearts, he knows I’m bi and from time to time we would have threesomes but we moved for college and the area wasn’t really into that so I told him no need to keep looking but clearly he didn’t stop. I know most girls will say well do it back make him feel like shit and trust me I do I really do but I can’t, I am deeply in love with him, I spent my teenage-mid 20s life with him. It’s easier said then done to just leave