I will start with the fact that my daughter was diagnosed with borderline personality disorder (BPD). These last few months it has not been good. She attempted suicide and cut many times. We just found her a dialectical behavioral therapist (DBT) and we're hoping that will help her BPD.
2 weeks ago she proclaimed that she was transgender. She created a FB profile with a male name, began wearing male clothes, and changed her Instagram name to her new male name.
I'm still in shock. I was on vacation last week with my husband do I didn't have to deal with the situation very much but it was still on my heart and mind. Today is my birthday. My daughter and I usually do special things together on this day and I had to practically beg to get her/him to go to Starbucks with me for an hour.

I'm frustrated and confused. The boy she has become is not like my daughter. The mannerisms and communication are different. It's a completely different personality. My daughter is vivacious and fun and this boy is quite chill. I seriously feel as if I'm talking to a complete stranger that physically resembles my daughter but is not her.
I don't know what to think and I just try to act normal but inside I'm screaming. I feel like I'm mourning a daughter who has disappeared or something and trying to act like everything is fine with a person I don't know but who gut-wrenchingly physically reminds me of her.
Obviously, I am going to have to go to a therapist. I'm also going to talk to her therapists. Will my daughter ever come back? Is this the BPD or is this new boy here to stay? I feel like it was so sudden. I thought we could at least discuss how to work through this but the boy doesn't want to talk. He just says " Whatever makes them comfortable. I don't want people to be uncomfortable."
She only ever showed possible signs of this in high school when her BPD was flying out of control, at least in my recollection. And she has always been a dress-up person. She adores putting on costumes and assuming roles. So when she went to her BFF's prom as her date and got a tux, we all just thought she was being sweet and funny because the boyfriend was too old to go to prom.
I'm so confused. What do I do? Mourn my daughter? Maintain hope that I'll have her back?