How do you truly trust someone after they hurt you?

Or do you believe it’s not possible?

My fiancé (been together 6 years) cheated on me once about 4 years ago. They didn’t have sex but she did give him head. Regardless of what happened, he cheated. We broke up for a while but while broken up, we talked. We had extremely deep conversations about life and our relationship etc.. needless to say we got back together. Been dating every since. We are now 21. I realize that I still don’t fully trust him. I’m scared every day he’s going to cheat on me again. Like I am starting to think I’ve been lying to myself saying I trust him. I come home from work every day and without him knowing look around to see if anything looked like a girl has been there. It’s that bad 😭 today we basically had an argument that ended w him asking if I trust him or no. I broke down and admitted I don’t think I do and I was very honest about what always is going through my brain. He basically told me he can’t stay with me if I can’t move passed what happened 5 years ago and that I need to make a decision. He said either I work on trusting him and stop bringing up the past or leave. And he told me he wants me to leave if I know I can’t trust him. My heart breaks cause I love him.. I really really do. And he’s done an entire 180 with his life. Liked seriously, turned his whole life around. I’m so stuck. How do I trust him. How do I get over this. Please. I don’t want to lose him.