I want my ex back.

Okay.. A bit of a story here, but please help me.

I am 16 years old in grade 10 in high school. My ex is 15, also in grade 10. (There is only 2 months age difference).

Some background on my ex girlfriend:
She was in a relationship with a guy, "John" for 4 months. She knew this guy since birth, and they have been friends for life. John is 20 years old.

Then I came in, and she broke up with him for me. (NOTE: This was her decision. I did not even ask her out until about 2 weeks after she broke up with "John.")

So I went out with her, for about 2 weeks, hardly anytime at all .

She decided to go out with her ex one day as friends. She says she wants to be friends with him still, which I understand. Don't forget that this guy has been in her life as a friend for 15 years.

So she goes out with him one night (I figured this out the day after.) I also figured out that he kissed her. She told me this, and she told me she backed away, and got mad, and stopped it right away. I didn't react to much because she told me, and I trust her. This guy has also been sending her love letter, begging her to go back to him, so I wasn't surprised when he tried something like that.

A day after she tells me about the incident between her and "John," we see each other in school. It is a normal day, but she has been quiet, and avoiding me since the incident between her and "John."

The school day is over now, and I am walking home. I get a text message, "we have to talk." This happens 30 minutes after a fairly normal day of seeing her. In the end, she calls me that night, and dumps me. I ask why, and she says she feels guilty about what she did with "John." She says the way I looked really broken when I figured out he kissed her. And she says that she doesn't want to hurt me anymore, so she broke up with me.

I saw her the next day in school, and we talked like nothing ever happened. I wanted to talk about the break up, and she knows that. I didn't want to talk about it in the middle of my english class though. So we were supposed to talk at lunch, she was "busy." We were supposed to talk after school, she was "busy." I aksed if we could meet over the weekend and talk but, you guessed it, "busy."

SO,
Does anybody have any idea of what may have happened? And how do I get her back?

Please don't tell me I am young, and have a whole life. I want her back now. Please help.

I should also point out that she really is annoyed by "John" and I am 99% positive she wouldn't go back to him. He wants 2 kids, and marriage. The girl only wants some fun, and a person to support her. She is, after all, only in high school. And John is in University.

So what happened? And how do I get her back?

Thanks for reading my novel of a story, and thanks for the help.
-J