I obsess over the negative things people say about me. Why??

ok, this fat girl at school always has rude comments to say about me and it hurts my feelings and makes me real sad and mad. I've never gone up to her to confront her about it. i just bottle it up inside, but just recently someone wrote me a note and put it on my desk, this is what it said, "(my name), one of (fatgirl's name) friends said that you're not about the sh!t you be talking. I bet you won't bump her and say sh!t to her cuz she will be dat Azz. Well, i took that very offensively even though i knew it was stupid and decided to talk to the fat girl about it. I found her in the bathroom and asked if she's been saying stuff about me and she basically cusses me out and says that YOU ARE NOT APART OF MY DAY, I DONT GIVE A **** ABOUT WHAT YOU DO. IF I'VE GOT SOMETHING TO SAY I'LL SAY IT TO YOUR FACE. i was happy that she didnt talk about me, but was upset at the fact that she basically cursed me out. i'm still bothered by this. my friends say let it roll off. but i jsut can't.