Men and women can’t be friends — and if anyone tells you they can, you need to set yourself straight. I think the problem started when you second guessed yourself and tried to accept her telling you that a guy she used to make out with and slept with was just a friend. He wasn’t, and isn’t. You have good instincts — use them more often! 😉 Bottom line is that he’s competition. Instead of getting angry at her, accept that fact — and that you let her go on a break, which is when she got together with him, so that’s not cheating — and work to make her only want you and not him. 😉
I hope that helps. Let me know how things go and if you have any more questions.