Oh one more thought.....

I think perhaps that you're more serious about her than she is about you. If she was head over heels for you and totally committed then she wouldn't be going out drinking with ex boyfriends! I know that if I was head over heels for a guy (or even just smitten) then I wouldn't be wasting my time on anyone else. She may never cheat but that doesn't mean her feelings run that deep.