Hi Always growing!
Welcome to our family! I'm so glad you have joined us.

To answer your question, BPDs are called borderline because their disorder touches the
borders of many other disorders. In the book
I Hate You Don't leave Me, there is a great chart which illustrates how BPD is in the center of a wheel shaped diagram with pie shaped wedges extending out from the center. Within each of those wedges are the names of the disorders that touch the 'borderline' edges of that center space. Narcissism is one of those mental illnesses/disorders that often accompany a BPD.
As you read posts from other members, I think you'll find narcissism is also a common theme here. My mom was an uBPD who was also very much a narcissist. I have read Understanding the Borderline Mother too, and for me the validation was pretty amazing. How did you feel after reading the book? It can be a tough read but very enlightening.
You are doing a great job of educating yourself, and I'm also glad you are in T.
Wools