Hi Fairyprincess
It is a sad point to hear your story that I can really understand because of mine. But the good point is that she will follow a treatment and that you will find tools here to know how to manage her behavior.
I am in a romantic relationship with a 28 year old woman. Her mother and my girl friend were really close when she was a teenager. Now she is doing her internship far from me in medicine and her mother has less and less contact with her (fear of engulfment or losing her independence I suppose). She is sometimes a hermit too.
The positive is that she is becoming more and more independent. She is experiencing real life without being babying by her mother or I. Your daughter may need to experiment true independence. You should be there just in case she is doing a bad thing (suicide... .), but try to let her the place she wants to have.
hope your the best,
Adrien.