I can totally relate to your post @Tosh012. My step-son has been diagnosed with DMDD and I have gone through many trials with him. His mother died when he was 6, and his father and I married when he was 8. He had multiple illnesses upon birth and had to be revived twice in the hours after he was born.
Having said all that, I wasn't prepared for the lashing out and rage that was directed at me. He would scream at me, swear at me, flip me off in front of our neighbors. At one point he attacked me physically when his father was away on a business trip. He has told me that he will kill me and his father in so many ways I have lost track.
When I first joined the family, I was outraged - how could anyone treat ME this way?? What did I do to deserve this? After all I had volunteered to be his mother, I had taken on this role and loved him, I...I...I...I... This gave me lots to drink over, and believe me, I drank and drank. My thoughts: If you had my kid, you would drink too! (Self pity is great fodder for drinking!)
It took me three years to realize that it wasn't all about me. He was broken, he was ill. He needed love even when he told me he hated me and wanted to kill me. Through his therapist, I learned that because of all the trauma in his life at such a young age, he would re-experience all of it as he matured. Kids don't have the emotional intelligence to rationally deal with things they don't understand. So they act out. And it's usually the person closest (family) that gets the worst of it. Alcoholics do the same thing, the people closest get hurt the most.
I had to change my thinking and treat him like someone who was very sick. I did not condone his behavior, however I also didn't rage back at him. I stopped taking what he said personally and realized he was acting out and needed help.
I know this is the most difficult thing to go through. Just remember she is hurting, broken, upset and acting out. I hope and pray she improves through therapy. You will get through this!! And one day, she may turn around and thank you for being there for her. As someone once told me about my relationship with my mother (we did NOT have a good one): just because she wasn't a good mom, doesn't mean you still can't be a good daughter. Your daughter needs you right now. Be a great mom (as you already are) even if she isn't being a great daughter...at the moment