How do you tell someone that you can't be their best friend anymore?

My "best friend" has been talking behind my back for a while now... I haven't given a second thought to it until today and decided to try and tell her and then started to type it but then wrote "... Nevermind" she started swearing and getting mad at me. I have decided that I no longer be friends with someone like that... she CONSTANTLY NEEDS to be THE CENTER OF ATTENTION! But how do you tell your best friend that you don't want to be friends anymore? Well as always... THIS IS AS GOOD AS ITS GONNA GET.