How should a Christian treat his enemy? PLS. HELP ME. ASAP. i want to kill her.?

cuz i have this girl who is my mortal enemy. She cheated and told the teacher. Since then she and her friends acted like i was the one at fault, she never really did own up to her mistake. It was four years ago then last April came our graduation and we ought to write letters to everybody and i wrote to her. I said i'm sorry for all the hurt and she didn't have to hurt anymore. i said it because i am a Christian, i knew i should forgive but still this drives me nuts. Because i what i really want to tell her is go to hell, *****. Anyways worse now because the way she is acting now makes me feel like i've strengthened her deluded thinking that i'm the one who is wrong. when she really is in every way. God i shouldnt have said sorry if i knew she would never own up. I can't stand the injustice. How should i act now? Should i tell her what i really think of her? Pls help me.. i so hate her and want to hurt please understand that i'm human too and being all goody doesn't cut it for me. there must be a way that i could appease myself thank you very much.