I can't quite get my head around your post...let me see if I've got it:
- You started dated a girl
- You met a guy and started cheating on your girlfriend with him. He is also cheating on his girlfriend
- You (rightly, I suppose, though it was a long time in coming) finally dumped your girlfriend because you had strong feelings for the guy
- He didn't seem interested in having a proper relationship, so you promptly went back to your girlfriend because it was the easy option
- You've carried on sleeping with and flirting with the guy, you even say you'd dump your girlfriend for him, so you're essentially going to carry on doing what you've been doing for a year now; i.e. stay with your girlfriend because it's comfortable and familiar, while remaining emotionally attached to this guy, who will take the opportunity to sleep with you whenever possible.
I'm sorry if I'm overreacting, but as someone who's been on your girlfriend's end of things before, I'm finding it incredibly difficult to feel sorry for either you or the guy (I mean, a one-off incident would almost be excusable, but this went on for a YEAR - did you never get a single flicker of guilt?). Obviously he isn't here for advice, but what I would suggest to you is to be honest with your girlfriend, tell her you don't feel the same way about her any more and finish things with her. Then never see the guy again because he's clearly happy to use you for sex while maintaining a relationship with his girlfriend.
Once you've cut your ties with both of them, start with a clean slate and try not to cheat again.