Hi Mom42,
We have had had so many different diagnosis from bipolar,ADHD, Depression,mood disorders and so many more .
I have been through exactly that. Including even a relative who called me when my son was around 18-19 saying that his behaviour reminded them of someone close to them that had schizophrenia. Not one therapist ever mentioned BPD, but I suppose it could have something to do with the fact that he wouldn't go long enough for them to truly figure him out.
In Valerie Porr's book "Overcoming Borderline Personality Disorder, A Family Guide for Healing and Change" she asks if this has happened to us. So, it is a very common thing with BPD persons.
I do recommend her book for people who have children with BPD although I feel like it is a good starting point and other books should be read as well. Valerie's book really helped me to understand my son better, she described him in perfect detail and gave me some tools to deal with him. I think her book is best for helping us understand, there are many more tools here in the site library.
Good news is that she is young enough that you still have some power. You can still say, hop in the car honey, we are going to see so and so (the therapist). By the time I realized there was more going on with my son than his simple diagnosis it was too late.
I wish you all the best. Buckle up, this is a journey you are going to have to navigate for a long time but don't despair. Try to look at it as the biggest challenge of your life. Are you going to stand up to it, or let it get you down? You can do this, we are here to help.
R