Why can't I let it go? Please answer!?

I know this is going to sound super childish but I am having so much difficulty forgiving and forgetting. Me and my boyfriend have been together 2 years now and like most couples we had problems , his ex's came into our relationship and messed up the trust between me and him , for awhile I was fine and just dealing with the betrayal feeling, but for the past 2 weeks I have been in emotional HELL I am so angry and I feel all the pain from what happened as if it happened yesterday, I feel insecure and emotionally vulnerable and that's just not who I am. I have never acted like this in a relationship EVER and I don't understand why I can't forgive and just forget about it , he is sorry and has done everything to prove that. He treats me wonderful now but I just keep getting these bi-polar mood swings lol I know I need to stop this because I'm starting to take it out on him, has anyone ever been through something like this? Can anyone give me advice on how to go about situations like this?