even though it probably hurts heaps, i think you should let it go. if you like this guy, look at what he is doing to you and the other girl. if you think that he will change if he breaks up with her and goes out with you, i think you may be wrong. poeple dont change immediately like that, they might for a week or so, but then they always go back to old habits, especially if he is older than 18.
hes not worth the trouble. if he is making you feel confused and helpless now, when he is not going out with you,
imagine how he is making the other girl feel?
she must be even more worried, confused and helpless than you because she has no idea if her 'bf' is doing anything with other girls. would you like to be feeling the way that girl is now? if not, then dont go out with him.