Why is my friend such a backstabbing *****?!?

i'm 16 so this stuff isn't kindgarten stuff. i hang out with like a group of 6 people, im pretty much a carefree person but my "bestfriend" is such a boy crazy whore. she always goes after the guys i've been with and is now going after my most recent one, i don't get it shes not even pretty is she just easy? she'd do anything for guys and instantly ditch me, she ditches me to walk past his house after school to get attention from him and he asked to finger her twice but had her period both times but she said to my other friends she would and said she was doing nothing wrong. i hate her at the best of times and i try to be a good genuine person but most of times being that person means your hell of a pushover that nobody wants. i feel so depressed i hate this feeling, am i being stupid? what do i do other than beating her ***. she also doesn't know i know about this, why did my other friends not tell me early ...