My best friend is terrible!?

Ok, She is always bugging me and is SUPER annoying. I know you guys are going to say stop being friends with her, But we have been friends for about....well since we were in pre school. I can't get rid of her now! I'm in 10th now so you know this has been going on for a while....One time we were in 5th grade and it was "Cat In the Hat Day" and I hated my hat and she MADE ME wear it the WHOLE day. :P Anyhow, When I'm sick at school and I need to go home she says, "If you go home we aren't friends anymore!" Which is the WORST because I feel awful....One time I had my 'Tacos' (Which is our code word for: Period) and I felt so light headed and I didn't want to bleed threw my pants and stuff and (This was 6th grade by the way) we were bored at recess, So My best friend, My other real close friend, and I wanted to play tag. Well not me....My best friend wanted me to be 'it' and I said "I quit I don't really wanna play..." She got all mad and said "Oh come on you always do this!" And ran away.... I hated that the most because she doesn't even know what I go threw. I just need to know what do I do about her....She is really starting to bug me and I need to stop her or something! Thanks! :)