How do I move on from a girl that doesn't like me?

Okay, there was a girl... I liked her a lot, and she said she felt nothing for me. I pretty much died on the inside, as I put in a lot of time, energy, and thought into her. There was this guy that I've really disliked for a few years, and I guess that God has a sense of humor; She likes him, and he likes her back. I know that it isn't her fault that she didn't like me, but I'm still in a lot of pain right now, and she's pretty much become dead to me. If I'm with a group of friends and she walks up, I say "Hello," to her to be polite, but say nothing else to her. Although she is dead to me, I just can't move on. The pain is ridiculous, and I've become even more depressed, shy, and negative than usual. I just... Can't even acknowledge a girl's appearance anymore, I just ignore every person that I'd previously found attractive. I know it's just a phase of heartache, but if I don't get over it soon, I may be socially impaired for a long time. What do I do?