I couldn't protect my girlfriend, im very upset?

A guy was hitting her on the butt and grabbing her boobs, so she pushed him away and then he grabbed her and started calling her a *****, and other names, she punched him in the stomach then he puched her right in the nose so i ran at him and pushed him to the ground then he got up and swung at me and i ducked and got him in the stomach then elbowed him in the face but it didn't knock him out. he then grabbed my wrist and sqeezed it really hard until i fell on my knees then he kicked my face and i was blacking out then my girlfriend came up to me to make sure i was ok, but i just walked away from her and i haven't talked to her since. i have been very depressed and even crying some nights. what should i do?