She was one of my best friends, and how she wants nothing to do with me.?

Okay, there's this girl who goes to my school who I, well, was very good friends with until today, until she took a comment my friend and I made as more something serious than a joke, which it was. She told everyone & me that she wants nothing to do with me ever and i'm out of her life. i think that was kind of an extreme reaction, but i dunno. But the weirdest thing is that she isn't mad at my friend, but only at me. She only took ME seriously although the joke was said by both of us at the same time. Please help me, cuz I really want her to be my friend again. FYI, I've said sorry many times, to which she ignores, and I've sent her an email and IMs, to which she also ignores. But she has also picked on me before about things that I have done, telling sometimes hurtful jokes in front of me, and I always forgive her, so why can's she forgive me? And why is she only mad at me? I just don't get it!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!…