I'm lovesick, please help?

I have known this girl for about 2 years now and i have never felt anything towards her until recently. She is my best girl mate and i feel physically ill when I'm around her now which sucks as I spend a lot of time with her. I can't ever be with her because she loves someone else. What is some advice to help me get over her? I am pretty down at the moment and haven't been going to lectures for a couple days now, but I am really determined to beat this. Sorry for the long post. Thanks in advanced for your help