It's hard to say what to do unless I were in the same situation. You have to ask yourself which woman is a keeper. If the "girl from your past" just seems attractive because you were with her or knew her from before, it proabably isn't a good reason to get with her. Every guy has girls from their past they probably wouldn't mind boinkin, whether ex's or good friends from HS or college, but when it comes down to it your current SO is probably more of a keeper than the fling from the past. Man up and do what you know is right.