Yeah, I figured that might be the case. In which case, you really only have two options.

One: you stay with your girlfriend and don't act on anything with your other friend- and honestly, not... dealing with each other face-to-face for a while really is the best option for that.

Or two: you break up with your girlfriend and try and give things a shot with this other person. But you can't go around thinking you can have your cake and eat it too.