Sometimes the worst thing about dating in high school is the knowledge that most high school relationships don't last. And yes, it still hurts. Your emotions are involved, so it's only natural.
She keeps blatantly flirting even though she knows it bothers you. This means you need to either put up with it, or break up with her. Consider that there are lots of other girls out there who WON'T do this to you. She should be worried about you leaving her, not the other way around.
There's probably only so much you can put up with, right? I imagine that you will grow tired of it and break up with her if she continues. She'll soon realize this is a quick way to lose her boyfriends.
“Inside every cynical person, there is a disappointed idealist”--George Carlin