girlfriend caught me in bed with her best friend, and she became REALLY violent towards me. she slapped me, which left a red Mark. and she screamed at me while crying. she even tried to toss my PS Vita outside, but I got it back. was that too much of her to act like that?
I think it was in a moment of rage. True, she acted a bit immaturely, but you sleeping with her best friend was bound to trigger that sort of reaction. It was wrong of you and I don't think there is much hope for saving your relationship. Just apologise to her and don't mess with her best friend again. Know that she's going to be really hurt because she's lost two people she loved. In the future, if you're not happy in your relationship and want to explore with other girls, break it off with your girlfriend as you've really hurt her.
You're a shit person, but violence is never acceptable. Her reaction was wrong, but I can understand the rage. You're despicable, and while you don't deserve to be physically harmed for it, I hope she leaves you for good, and that you stay alone until you learn how to be a decent human being.