Hi
Lynnie 
I'd like to welcome you to BPD Family. I'm glad you've reached out for support, relationships with pwBPD (people with BPD) are difficult at best and it's important that we take care of ourselves first and foremost.
As for how you can have a conversation with her about the diagnosis that she hasn't accepted yet, it sounds like you can't, at least not in this moment.
What you can do is learn all you can about BPD, we have lots and lots of articles here and wonderful parents who are members, posting their experiences and getting and giving support.
What led to the diagnosis? What are the behaviors you're seeing that cause you the most concern? Is she living with you?
Sorry for all the questions, it helps to get more information so we know how to best support you.
Again, welcome here - I look forward to getting to know you.
~ OH