Cheated on and got back together before I healed.

I don't know what to do anymore, he had cheated on me and ended our relationship claiming it was so we could start over and fix our issues with communication, meanwhile he was still talking to the other girl, his ex. It was months until I found out, she told me cause she couldn't stand him not making a decision yet and thought I had to know. I suspected it for a while but continuously made excuses for him telling myself that he would never do that. I text confronted him since I was at school, he continued to blow up my phone apologizing and saying that he screwed up. I read it after hours of ignoring it and he went to find me at school, we talked for a but but he was in so much agony I wanted to comfort him. I told my friend I didn't know what to do then, she advised me to take time to myself and think, I loved him and it had seemed like he loved me.
We continued to talk, after a few months we were basically a couple without a title. A few months after that we got back together. Its been 6 months since then and although I never got mad at him, I haven't let go of what happened and my mind replays it often. I know this is unhealthy but I can't stop being triggered about it and it makes me really insecure. We've never talked about it too much cause he starts to cry and at that point we just can't talk. I still don't know why he did it or for how long it was going on.
I love him and I do want to have a future with him. For the most part I do trust him. I just don't know how to move on and "let go" of what happened last year. Help me please.