My friend is trying to steal my boyfriend!!! help!!?

Me and my boyfriend hav been dating for more than two years now.so now that summer is here my friend(or enemy) is trying to get me out of the picture.Shes liked him for quite a while now and shes told me herself that she has feeling for him.Truth is she tries to hard to get him or any hot guy that walks past her.the funny thing is shes only had 1 bf in her life but i dont blame the guys. shes actually a real ***** always complaining, and thinkin we're freaks and that shes the"normal"one around.so shes been txtin him a whole lot and chances r shes been flirting with him and shes even flirted with with him INFRONT OF ME!!! so wen my friend tells me shes been playing all lovey dovey I LITERALLY feel this anger in my chest wanting to pound her face!!(srry if im being real violent but im in fury) and shes asking my best friend if he and her will hav a chance and i know for a fact that he DOES NOT like her. and i aint just assuming this, my cousin is his best friend since kinder and he tells me the things he thinks of her and I AM JUST RAGING WITH ANGER!!! help :(