I know excatly what you are going through...
It sounds like you have a case of unrequited love, something i suffer from also. When you feel something strong towards some one that does not share these feelings back...
The out come of your whole day depends solely on whether of not you get to talk to this person that day. When you live in your own little world where you make it seem like some how you can get her to feel the same way, and every once in a while you realize how pathetic you are.... for still trying...for not getting it. You tell yourself you need to just find some one else...
You try and put your self out there and what do you get? Nothing.. more rejection. So you find yourself back where you were.
Its not easy, especially when you start to see this person you feel so strongly about try and get involved with other people. In fact its Hell.
But the truth of the matter is this is an extremely common situation... It was been writen about in books, and there are more songs about it then you could possibly count. Like Pearl Jam's song "I Got Id" for example. "And I stay in bed. Oh, little ive seen there. If just once I could feel loved, oh, Stare back at me." is a line out of the song that shows how the he yearns for her to be next to her, and for her to look back at him the same way he looks at her.
One way I try make it not hurt so much is by trying to put myself in the other persons shoes. What would you do if you knew some one who felt something for you, when you felt nothing back? Should you have to go into a relationship with this person just so you don't have to let them down? You should never be forced into a relationship with some one you have no feelings for. Its bad for everybody.
But no matter what, Its still hard. Especially if you still keep in contact with this person... I guess all you can really do is wait it out. Try and stay sane.