this is what i would tell you -
1. i think you're sad because you think "giving up" is your only option and it hurts. you want to keep hoping. well....hope all you want. knock yourself out. unless one or both of you are planning on going somewhere far far away permanently, there is always hope. but....you need to put her on the backburner. chasing won't help. now, putting someone on the backburner and giving up are 2 different things.
2. people talk about how bad oneitis is and all that, but it's hard to not be that way all of a sudden. i don't know why people write stuff like, "just get over your oneitis" as if they never went through a phase like that. yeah, whatever. it's not easy. don't get down on yourself about it just because other people here are trying to make themselves feel big at your expense.
3. having said that, the easiest way to get over a girl is to find one that is attractive and turn your attention to her. now, ideally it is multiple girls, but in your case, one will suffice. there are plenty of girls out there who may not be as pretty as the one you like, but may be better people. give them a chance. talk to them, get to know them, laugh with them. it will take your mind off of her.
4. if it never works out, you will get over her. might be soon, might be a while, but it has to happen. it has to. nobody i know has spent his entire life pining over one chick that they never even had a relationship with. never. maybe an ex-wife or something. but not this. you will realize one day that she's not all that, and will wonder why you were so hung up like you are now.