You chose to get involved - and you can ALWAYS choose to get UNinvolved, whenever and wherever you want. It's totally up to you. It sounds like you feel trapped. I think you need to stand up for yourself - don't see him until he breaks up with the girl. I worry that it will just hang over you and never change. It sounds like you're ready for an exclusive relationship, the question is if he's at the same place or not.

take care!