The only bit of common sense I can offer is the one of caution. If a woman is willing to end a relationship that has progressed to the point she is going to be married, and is willing to leave that person; the same can be said for her relationship with you. It's the cheaters' dilemma: If she cheats with you, she will cheat ON you. You should remain the good friend to BOTH parties and things will work out as they intend to. Also, don't be the rebound guy if it ends badly.
If you were really in love with the girl you wouldn't be asking us what to do, you would be going after her. I'm assuming you don't really know or socialize with the guy that she's engaged to..so screw the guy. Go get what you want, that's that. Now..if you know the guy personally..yeah that's wrong.
The way I see it, if you don't know him...games on. If you do, show respect and try to get over her.
But..as someone else mentioned...if she's willing to cheat on a guy she's engaged to, one has to wonder if she will do the same to you at some point? You better be sure she's going to return the love so that won't happen. If you even "steal" her away from her fiance that is.
OP:
If you're seriously asking this question, you obviously don't love her enough to respect her wishes.
She's in love with someone else. It's not her problem, nor her responsibility that you are in love with her. She has no obligations towards you, and you have no rights to intervene in her life. Period.
What to do? Keep it to yourself. The feelings will subside, and you'll eventually meet someone new. That is, however, if you lét it. If you start obsessing, you'll just be alone... Forever.
dude, I was in love with this girl form high school who is married...it suckkkkked, best option is to move on, find someone else and hope maybe you two somehow end up together, just dont count on it
I'm going to close this since it's touching on some pretty questionable territory. I'd also please ask people to be a bit more considerate and careful about the sort of advice they give out in the future.