Sweet mother of all that is holy and pure.
I have seen this so many times. Once first hand. The girl has picked her mark, used him up, and is still too sick to let him die in piece. She doesn't want to close the door on any possible attention she could get, so he gets strung along.
Sorry to talk about you like you weren't here, this kind of stuff really gets to me because it is obvious to everyone but the one in the middle of it, as this thread shows in spades.
Look at it this way, a person either wants to be with you or they don't. You obviously want to still be with her, buying gifts, just barely managing to not contact her for an hour, filling your mind with it until your grades go south... This is how a person behaves who wants to be with another person. What she is doing is the opposite. She is doing what a person does when they want to be free of a commited relationship.
That is it. There is nothing else to talk about. It is over for her except whatever she can squeeze you for.
Dude you can't imagine how effed up this is going to look to you a year down the road after putting her out of your life. You are going to thank yourself and feel like you owe us an apology. Which you don't. This is just something a person should go through once in their life to know what the meaning of manipulation really is. Unfortunatly no one can really give you advise that you can use right now, just stuff that once you are on the other side of it you can go back and read and really understand in retrospect.
How many posts have told you the same thing here and you go and buy her a gift? And you would take her back in a second if she came back. Please try to put yourself in her shoes and imagine you saying to someone else what she has been saying to you. What is it you would be trying to do if you were saying what she has said? Get slowly and clumsily out of a relationship is what.
You are a nice guy, you deserve to be with someone who is also nice.
.......... 
