I need to find strength to walk away.?

The Basics: I've been with this guy for almost three years. I've been 100% faithful, he has not. There's always another girl popping up here and there...on top of that, he's broken up with me so often, yet now I am the one getting fed up. I am not here bec. of love, I'm here bec. it seems "easier" to stay than to face things alone...I want to walk away. I just don't know when I'm going to (if I ever will) get fed up. I know the stuff he pulls isn't ok, yet I can't make myself leave. I've cried too much, been lied to more times than I can count...I wanna get out. I just feel so scared. I know I am attractive and that I deserve the best. Everyone deserves the best. I don't want to hurt anyone...Any comments/ideas?