Does my boyfriend need to know?

I just got engaged. While my fiance and I were broken up and not speaking I slept with someone else. I thought we were over and never going to speak again. Now the guilt is eating me up. He said once he didn't know how he would take it if I had done something like that. He said he would have moved on. Now we have a new lease on life and our relationship is going great. He said some things shouldn't be told to anyone but God, like something that would make me leave him. What do I do?