Most Helpful Girl
there's a quote i know that says something like "If you fall in love with someone else while you're with another person, it means you were never really in love with the first one."
im not sure how accurate it is, but if you think about it, it kinda makes sense. my advice to you is to think about your girlfriend and then the other girl in her position.. how different is it? is it better or worse?
if this was me i wouldn't want my boyfriend liking someone else and dating me so if you really do like this other girl, you should consider breaking up with your girlfriend before doing anything else.
Most Helpful Guy
Then you need to break things off with your girlfriend, and pursue her.