Foreva I am so sorry for your dear daughter and this painful experience she is going through.
I know when things happen we just want it all sorted and it is so hard but it is good you are looking for help and obviously a caring mum.
Can I just tell you my experiences, while also saying that I have not adopted a child (yet) and may not be the best person to speak to you about this, but I just wanted to share what I know personally.
I have suffered from anxiety. Mine came in my early 30s and lasted probably a couple of years. My experience was that I knew I needed help and I went to my GP, described my 'symptoms' and he referred me to the local hospital where I got CBT (Cognitive behaviour therapy) for free. The CBT was VERY helpful and sorted the problem out for me.
I am not adopted and I have no idea where the anxiety came from but it was very strong and unpleasant and I was very grateful to get help.
I would really recommend getting some help and I would try your GP first (but am happy to be contradicted if someone else can give better advice).
Other forms of therapy (than CBT) might be better or might help more. But I really agree it is better to get some help at this stage and be involved in it and coping strategies.
A friend has a child who has a great deal of anxiety. I am pretty sure she used this book:
What to Do When You Worry Too Much: A Kid's Guide to Overcoming Anxiety (What to Do Guides for Kids) by Dawn Heubner. I think it was helpful.
www.apa.org/pubs/magination/441A314.aspx
My daughter seemed to have some anger problems a while ago and so we tried the book
What to Do When Your Temper Flares: A Kid's Guide to Overcoming Problems with Anger (What-To-Do Guides for Kids) by the same author
www.amazon.co.uk/What-When-Temper-Flares-What-/dp/1433801345/ref=sr_1_2?s=books&ie=UTF8&qid=1357866765&sr=1-2
The book was good but did not solve our problems! They were sorted out through working through them BUT I think my friend found the book helpful so I just suggest it. My friend also talked to the child and adolescent mental health services and they were very helpful and visited and worked through things with her child (who is the same age as yours).
I wonder if adoption services offer anything?
I am NOT sure that this type of book would be helpful in this situation (although you might wish to read it just to see the ideas, the anxiety one not the anger one).
I do really think therapy will help and actually maybe this behaviour and concerns is uncovering some issues connected to the adoption and so to try and gloss over the worries and make her feel better in the short term would be the wrong kind of solution!
Personally, I think fear is a very strong emotion and very hard to control, talking about all the issues in an age appropriate way and then getting some help to deal with controlling those fears might well be more helpful than simply trying to 'deal' with the symptoms. having said that I am not sure if CBT totally does this, but actually it may be done in a whole variety of ways and I would certainly make finding out a priority - hopefully it will probably turn things around in terms of how she feel.
Just my opinion, and as I say I am not an adopter so feel free to ignore me.
Really hope you help your DD to find the solutions she needs.