Shay, I applaude you for being so in touch with your insides and for NOT going ballistic...I empathize with you on how it feels to be in or around a confrontation. I used to wish I could disappear. Then when I was married to my ex I found myself starting to feed on the adrenaline both consciously and unconsciously and it almost cost me my life during an "out of control" argument over a plumbing problem. I found myself on the floor in my hallway with my husband sitting on top of me with his hands wrapped around my throat. So ask me now how I feel about escalated anger....I went to anger management to do two things: Learn how to de-escalate a situation so I would not put myself in that position ever again and also to learn what healthy expression of anger/boundary violation is. I was not allowed expression of most anything as a child so as an adult I went to extremes and most of the time it was with the aid of booze, drugs, sex, whatever artificial outside means it took to get past my fear. It has taken alot of change in my life -some of which are going to AA, being in therapy, a divorce, losing acquaintences pretending to be friends, letting down walls, taking risks in the face of failure, being okay with calm instead of chaos, learning how to talk about MY feelings to express myself as opposed to YOUR actions. It is NOT easy in the beginning; it feels like new shoes that don't quite fit cause they need to be broken in....I am still progressing. I will never be perfect but I do not give up and hate myself anymore.
Not having children myself I don't feel qualified to give any "advice" or thoughts on how to handle your girlfriend's daughter's outburst towards your girlfriend. I think some of your actions would depend on what "parenting" rights you have been given and what support you would get from your girlfriend to back you up. There are so many unknowns in the confrontation you described too so.....I will abstain from further ideas.
Take care of you!

Love, hope, and strength,

Tooter (Pam)