Thanks all. Flow I have read your comments on the other thread and they are very helpful. 
But I'm struggling with feeling WORN OUT with all this, despite taking some time out for myself (I can do this when dc's are at school). My heart feels really heavy.
No matter how many books I read or how much I know about brain changes in adolescence, I still can't seem to get my head around the fact that DD is deliberately destroying our happiness as a family because she wants to. She isn't ill. She isn't damaged. She has been loved all her life by lots and lots of good people. She is healthy, clever, has friends, has a nice home and yet, and yet, and yet.... all she wants to do is make a normal, happy family life impossible for us. I'm really aware that life is short, that illness can take any of us at any time, that we deserve and should strive to be happy. I lost my dad a few years ago, but I still have the rest of my family, my health (sort of), DH. We are solvent. We have a secure home. We have NOTHING to stop us from being happy - except dd, who is doing all she can to make life unbearable.
If she was anorexic, lonely, had ASD, was depressed, had learning difficulties or a developmental disorder I could understand her having a massive struggle to cope with growing up, and would accept what's going on as unavoidable fall-out. But she's none of these things. When she's allowed to do whatever she wants she is actually very high spirited. Happy. Every single confrontation and crisis that we've had to deal with has arisen because she WON'T DO ANYTHING SHE DOESN'T WANT TO DO. And when she is thwarted in doing what she wants to do, she becomes malicious, vindictive, violent. She is also frighteningly manipulative.
How do you mentally deal with someone you love behaving towards you - for years - in a way which is stupid, abusive and cruel? Sorry to be saying all this - I know that anyone who has struggled with a really difficult teen has had to come to terms with this aspect of the experience. But I can't come to terms with it at the moment. Neither can DH. I don't know if either of us ever will.
I feel like I'm experiencing a sort of bereavement.