How to trust after a betrayal?

I have been with my boyfriend for 2 years, and we have known each other for 5 years. I am so lost right now. When him and I started to date he had just went through a break up. He wasn’t over her yet, and I didn’t care. I didn’t want to be friends with him. (We dated shortly, about 4 years prior) We dated and fell for each other so fast! We were living together and after about 7 months we moved across the country together.

But so did his ex! She would NOT let him go. She kept contacting him and making our relationship weak. Finally he caved and was curious so he started to talk to her. Then he started visiting her. And then after many problems, she apparently got kicked out of where she was living, so what does he do…MOVES HER INTO OUR HOUSE! I couldn’t take it anymore. I fell apart! I got a plane ticket, sold my car and moved back home to my parents. It was only a month and her and him drove back down here and she went back to her parents and he somehow convinced me to go back. I never should have b/c our problems were not fixed, and she had left things ALL over the house for me to find. I fell apart so badly. I came home after being out there a month. I came home December 2006. It took me until July 2007 to be completely over that pain. He has changed his life so much that I barely know him anymore. He hasn’t talked to her in 9 months or so. A lot has happened and I am so in love with him. It is the greatest feeling in the world to see someone do everything they can to show you how much they love you. By why isn’t that enough? Why am I in so much pain? Why can’t I see his changes and get past it all. I forgave him, but I don’t know how to move on. I am finally asking for help. But who can help me?!