Two words came to my mind after reading your post. DUMP HER. My god. Not only would I be really pissed at her, but I would drive the 3 hours to her school first, with a baseball bat as my passenger, beat the shit out of the guy screwing around with my girl, and then dump her.

Bottom line is, do exactly what you said you didn't want to do. Her living it up for senior year shouldn't consist of her screwing around with other guys. That's perhaps the dumbest thing I've ever heard. You should tell her that her behavior is unacceptable, and if she wants to be in a relationship with you, to end that bullshit. If she doesn't, tell her your done. But like I said, had shit like that happened to me already, I would've dumped her right away. You gotta hold on to your dignity man. And if you let her pull that shit on you, not only will you be her *****, but you will have no dignity. Good Luck