How do I let go and move on?

Ok, here's my situation. There's a guy that i met about seven years ago. When I first met him, I liked him,a lot, but I didn't know how to balance a relationship and getting through my college degree. (I choose the degree.) About three and a half years ago we bumped and connected again. He told told me the he was engaged to be married (I felt the lost.) Well Basically we decided on just having one night before the wedding. Needless to say, We've been together ever since (3 and half years), BUT he's married! I want to let it go, but apart of me doesn't. I keep telling myself, if he wanted you he wouldn't have married her.I need help. i know it's wrong and i want to let go, but I don't know if i can. I have tried to stay away, but when he calls that all changes. We've tried to be just friends (he's a person that i want in my in life - a dependable friend) and have fail. i fear the only way is to cut him loose completely. help!