Hi orthoRN,

I'm sorry to hear that. I'd like to welcome you to the group, it has to be really hard to see a teenager go through that. Black and white thinking is not uncommon with teenagers, in fact that's a reason why professionals don't like to diagnose teenagers.
To answer your question, when does it get better? I think that it starts getting better now, you registered on the site, it helps to talk to others that are going through something similar, you're not alone, we can offer you guidance and advice.
I'd suggest to read as much as you can about the disorder, there is a fundamental logic to the behaviors, it helps to understand why your SD16 acts the way that she does. It's a something that she's going through, it helps us to rate ourselves from our pwBPD and understand that it's not personal. Become indifferent to the behaviors and depersonalize it.
One of the most important things is self care, it's more important when you a family member that suffers from a mental illness, you don't want to burn the candle at both ends, you feel better and less emotionally distressed. Are you seeing a T? ( Therapist ) is your H supportive with SD16?