Major drama problem, my so called "best friend" is such a *****, can u believe wht she did to me?

okay so me n my mom got into a fit n she scratched my chest n she left a scar so i told everyone tht i got into a fite but since my best friend (her name is barzman) was close wit me at da time i told her wht rele happened. then a few days later me n barzman were fitin n she threatened to tell everyone wht rele happened to my chest juss cuz we were fitin. so i realized she wasnt a good friend so i tried to stop bein friends wit her n she was like fine since were not friends i cant keep ur secret so i had to stay friends wit her, so basically she was blackmailing me. so i realized i had to make it so she wouldnt want to be friends wit me so i tried to make myself seem like a bad person so i told her i had wit some guy (im 12 btw), i told her i got arrested for grand theft auto, i told her i cut myself (which is tru (i cut myself because it pisses me off of how miserable she makes me)) so she was like wow n w/e. so one day we were tlkin n she was tellin me about how she wrote down all of my problems and made them into a list n she was like if u eva threaten me whn were fitin ima tell everyone all ur secrets. so here comes the question... WHAT DO I DO? HOW DO I FIX THIS? HOW DO I GET HER TO KEEP HER MOUTH SHUT??