The exact same thing happened to me, except with a boy. We were seeing eachother, he asked me to be his girlfriend, and the next day, he chnaged his mind. I was heartbroken. I stopped talking to him and wallowed.
It turns out that he regretted it (we had mutual friends) but by the time he got in contact with me again, I had got over him, by focusing on his bad points, going out and flirting with guys, reassuring myself that men found me attractive and it was HIS loss.
I ended up getting my own back because I agreed to go on a date with him to the cinema: he paid for premier seats, bought all the food/drinks AND took me out for dinner afterwards, three course meal etc. Then, when he took me home (a good 45 minutes from his), I said "Cheers" and as he leaned in for a kiss, I walked into my house and slammed the door in his face. And I never spoke to him again.
Anyway, so. Focus on her bad points. Go as far as to make a list (purely cons, mind! There are no pros about this girl as as your concerned!) and just make yourself realise that she is not worth it.
Go out with your friends, and I know you said you always think about this girl, but just get her out you head for one night. Maybe actually getting with a girl as opposed to just talking to one will help you forget about her. You need a rebound to get over her, me thinks.