you're damned if you do and damned if you don't.
1) you stay with him - he dumps his girlfriend and dates you - you have it in the back of your head that he will do the same to you.
2) you stay with him - he never dumps his girlfriend - you eventually grow up and get out of a potentially damaging relationship.
3) you break up with him - you regret it because you care about him.
4) you break it off, and move on, and eventually become the type of person who doesn't get wrapped up in other peoples bullshit.