Step-daughter threatening entire relationship

I am in a real quandry. Last Thursday my step-daughter (16 yr) and my oldest daughter (21 yr) got into an argument. I kept them separate and took no sides. Then my girlfriend got home and we were trying to sort things out. Her daughter refused to follow directions (to come into room) since my daughter was there........it all began as a fight. Eventually I sent my daughter out of the room and tried to address her daughter. She kept screaming, cussing, and stating that I hit her all the time, treat her bad, etc. I hit her once 2 yr ago and felt so bad since that I have stayed away from her. She follows NO rules and lies to her mother about what is really happening. She also plays on guilt her mom feels. So she was yelling at her mom that she always lets me do whatever. I lost my temper and yelled that she is exagerating and needed to quit yelling at adults. She told me to f..off in so many words and dared me to make her. I realize I should have left the room. But I didn't. I went around the be and grabbed her face in one hand to make her look at me........she jerked away and I attempted to grab her arm (all in anger) she then punched me in the eye just as her mother jumped in and shoved me away. Her mother became a lioness and shoved me many times to get me away from her daughter. Once her daughter left the room (she still stood just outside the room to watch it all) her mother (my lover) shoved me several more times.........into furniture, on the floor, etc. The anger she felt/showed was more than I have ever seen. It scared me and humiliated me in front of her daughter. Immediately my eye became swollen and black. I have bruises on my side and back. Though I see that I should never have laid a hand on her daughter..........it is being treated as if this was all my fault and her daughter should have no repercusssions. My heart is breaking. How can my girlfriend not see that grabbing a "child" is not the same as striking someone. Her daughter has always had problems with boundaries and this is making it even worse. She refuses to appolagize and even seems to gloat about it. My kids are all confused because they have never seen violence and would NEVER strike a parent/adult. My heart hurts. My girlfriend feels hurt that I violated a trust and hit her daughter (though it was really grabbing). What do I do? I have cried about this for several days now. And part of me just wants to run away. Leave. Even die. Yes its excessive. My love for my partner is everything. I recognize I don't manage teens well. I expect boundaries. I have been through this with 3 other teens and NONE of them would ever strike me. Now the teen is mad she had her cell phone taken away as she was "defending herself". Any suggestions would be appreciated. Even the ones I might not like.