Why did she break up with me?

She broke up with you because you didn't meet her expectations.  I don't know what those might be, and you probably don't know either.  She might have expected more of your time, like not just Saturday nights.  Maybe she expected more phone calls during the week.  Maybe she wanted more presents, or more dinners out.  Or maybe she just got tired of you.  Maybe your stories about the high school big game got boring after she heard them a few times. 

Be stable in your emotions but not cold. Try to stay "friends" at least letting her know and feel and trust that you will be there for her. Don't over-apologize but do sincerely apologize for leaving her alone etc. Make her feel you are on her side, you agree but love her and believe she's the one, that you're upset but can handle it, because she needs someone who is devoted but won't lose control of their emotions (since she probably does this enough to her self for the both of you), it means she can trust you to lean on you. 

Also take care of yourself enough, she will be attracted to mothering you most likely but becoming pathetically gross or lacking in school/work isn't attractive. Make her see that you are capable of being strong and support for her.

You can find more tips on the magic of making up, http://www.freebreakupcourse.com/

There are a lot of ways to win a girl's heart, but you have to know what you want first. If you really love her, you have to swear to yourself not to hurt her third time around -- if you cannot do that, then give her some peace and leave her alone. Make her happy.

To have her back, win her trust. You have to make a lot of sacrifices to show her that you love her. It would take time, and there's really no assurance, but that's the price that one has to pay for hurting someone.

Magic of Making Up tips will help you more to understand how to handle the situation: http://www.freebreakupcourse.com/

You really need to stay busy with your friends and find fun things to do. Occupy your mind with books, movies and people. Go out, go for a walk. Physical exercise is very good for stress. 

You need to go to counseling if you can't accept this breakup, because he has moved on and doesn't want to be with you. You need to bounce back and get out there and have fun, you are so young and you have your whole life ahead of you. 

Otherwise, you got to find ways to handle the pain of break up. You can also seek tips on winning him back. Magic of Making Up can help you, http://www.freebreakupcourse.com/

Because you a needle dik.