I feel like I am being pushed away

I love my fiancee. However he has cheated on me with someone I thought was my friend. It happened 3 years ago and I found out cause I seen it not cause he told me. This scares me what other secrets he has. There is a lot of time we spend together but we did back then to. I don't know how to tell him this is eating away at me. He knows I know. I really need to talk to him cause I feel like I'm not sure if this relationship is strong anymore. There is a few other things that I will probably post but this is the main one. I really need advice on how to talk to him about it without him getting upset or just wanting to end it before we get a chance to fix broken pieces. Please help me if you can.