Carole, I was chatting to a friend last night. She has studied psychology and is dd1's best friend's mum, so she knows my dd well.
She thinks that teenage children hit out at their parents because they know that they will take it and still love them no matter what. She is a special needs teacher and has seen pupils who are delightful all day in school hit out at their parents in any way they can. Like a complete personality change when the mother came in the room. She thinks that a teenager will think about it and find the thing that is going to hurt you the most, and use it to beat you over the head.
In your dd's case it could be that she knows this relationship is important to you, so she hits out at that. My dd still insists she can't stand my dh, even after 7 years together, and as everyone who has met him will testify, he is the loveliest man and a wonderful stepfather. She knows if she criticises him it hurts both of us a lot.
It would be best not to let her have any influence over your relationship with him, he is special to you and you should be allowed a life of your own. Soon she will be grown up and no longer your responsibility and I don't see why you should lose the chance of love and a happy future yourself to satisfy a whim of hers.
I often find when I am struggling with my dd that what she really wants is for me to be firm. I am reluctant to discipline her since I was hit a lot as a child and verbally ridiculed by my mother and I would hate to think of her feeling as hurt and abused as I did. So I end up being too soft with her. On the odd time when she really goes too far and I ground her, she suddenly becomes pleasant and approachable - it's quite bizarre really, as if she were crying out for a firm hand. Perhaps they need the feeling of safety when an adult says to them, 'Listen here, I am the adult here and I make decisions about what will happen to all of us.' It takes a whole lot of responsibility off their shoulders.
Your doctor's not yet making a diagnosis sounds quite level-headed, she evidently wants to check out all the possibilities and make sure the treatment is appropriate. What will happen in the meantime? will dd see other specialists who will help your doc formulate a diagnosis?
I must admit also that my dd has been a lot better since she cut down her internet time. She used to spend hours and hours on msn but now she has cut it right down she fills her time with more relaxing hobbies like cardmaking and reading. I think we are gradually getting there with her but it has taken a long long time and my poor dh has almost had a breakdown in the wake of her illness and other events. I'm looking forward to the day she begins to grow up and see my husband for what he is. It will have all been worth it if she realises she loves him one day.