Honestly? you should move on. Yes I will be honest and say you have the responsibility in this in that you agreed to have sex with him and did so. That part, I don't know what you should do about but he, quite simply, used you for sex. That he said he won't/can't leave his gf for you, says basically it was sex and he's a cheater and is ok with having had sex with another girl and cheating on her while he's not wiling to break it off as if he loves her but can't keep his junk in his pants? Sorry but even if you loved him at one point, would you even want a guy like he clearly is now? If he's willing to do that to her, at some point, he'd do it to you anyway.

Move on... you'll get over him.