She sounds a lot like the girls that age that I taught in public school. If they showed too much interest or got too excited, they felt vulnerable and weak. This is her defense mechanism.
Bless you if you go through with this adoption, I know after teaching that age group that I could not do it. As much as I loved my students, I couldn't bring myself to adopt any of them. (Not that any were available for adoption.)
I don't have any advice for what you should do, just my 2 cents on possible motives for her attitude and behavior.
I ran into a former student the other day. I told her hello and used her name. She said she was surprised that I remembered her. I said, "You were a good kid (not really but had potential) and I always remember the good kids." She looked at me and said, "Well, I had a baby."
It was like she was telling me that I shouldn't like her and that she is not good. "I dare you not to like me" kind of a thing. She was like that as a 14 year old too
It made me sad to see kids everyday that treated adults like that just so they could justify not getting attached.