Fighting is for people who aren't intelligent enough to say anything smart so they resort to kicking someone's ass to try to prove a point and that point ends up being that they're a fucking loser for wanting to fight in the first place.
If she has a man, who cares, there are plenty of other girls out there for you. It wasn't meant to be for a reason. As for the slip up, no big deal. You don't have to ever mention it again. It will pass and people will forget all about it.
I liked a guy back when I was in high school but he had a girlfriend and I felt like shit. Everytime I saw him, my heart would flutter and all that lovey dovey shit but you come to terms with things being the way they are and you accept your fate. Now, I woudn't really care if he asked me out because I'm a happily married woman and don't have feelings for anyone else but my husband. He has me on Facebook and, well, I laugh because the girl he ended up with is nasty, lol.
Life goes on.