Why cant I just let him go?

I am in a relationship that I know doesnt work and isnt good for me. My "bf" for lack of a better term and I arent exclusive and I know he see's other people. He manipulates me and lies to me and I know it and yet I cant seem to get my heart around the fact that I need to just get out of this relationship. We have fun when we are together and I really like being with him. What the heck is wrong with me.