What would you do in this situation?

I've been best friends with this girl, lets call her "C". She seemed like a good friend last year, but this year I'm trying to "outgrow" her. I don't like her additude, or her personality. Sometimes she can really just be a ******. Nobody at my school likes "C", but today, during a math quiz, I realized that when i looked at her texts (we switch phones and look through texts on each others phones, don't ask I do it with all of my friends) I saw when she texted the "king of secrets" (that is what I call him at school, he tells all secrets and seems to consume all the "gossip and rumors and everything like that"), she had told him who I liked! Let's call this boy, "X". So "king of secrets" told "X" and probably even more people. "C" even told "king of secrets" GIRLFRIEND. She's also called me a B**** and a ****** (Yahoo answers won't let me type it. Replace the "M" in maggot with an "F"). I don't really want to be friends anymore. She's done this to me before too. And to my other best friend. If you were in my situation, would you de- best friend her? What would you do?