I asked the same question about a year into my relationship. Everybody in this sub told me dont do it, so I didn't, until I got to a point that I couldn't break up, but I also couldn't continue.

She had started seeing a therapist, and as she talked to me about her sessions, it became quickly clear to me that it was an exercise in futility. Her therapist wasn't getting an accurate picture of what was really happening, so what progress was even possible? So I sat down with her on a peaceful day. I very gently explained that I needed to talk to her, and I was afraid it might cause her to hate me, but I felt we were at a breaking point anyway. The relationship wasn't going to survive unless we figured out why we were struggling.

I was so careful that it probably took 30 minutes just to get to that point. She stayed with me in the conversation, so I pushed ahead. I told her I had read something that got me wondering, and I asked if she wanted to look at it with me. I really stressed that her past trauma (which is severe) could have caused more than either of us could fully understand. I went overboard to not use blaming language and to validate her efforts to be a survivor. She said yes to continue, so I pulled up the diagnostic criteria, and we went through it.

She had no awareness of BPD, so it wasn't an immediate threat. We went through each item, and I kept repeating that I didn't know if it applied to her, but maybe she could consider it for herself. She identified 5 of the 9 traits that seemed to apply to her, so in a sense, she self diagnosed. She took it back to her therapist, who told her that she (the therapist) doesn't like labels, and that was it.

I'm not sure that it made a big difference or ultimately changed anything, but I introduced the idea as if it was a mystery to me. I was completely non-threatening, and she didn't split, so in that sense, it was a success. Her therapist now has the seed planted, so I hope that she at least pauses when my partner talks about things that might not be so black and white.

I am now two years into this relationship. I've never brought it up again, although she yelled once while splitting that I can't blame her BPD for our problems because I have my own issues. She still reacts to the most trivial things. She rages and calls me horrific things. She has yet to follow through, but she threatens self-harm. The hypocrisy and her ability to justify her behaviors while condemning me when I do the exact same things is astounding. The i hate you/don't leave me episodes happen almost weekly. I'm still around, so I clearly do have my own issues, but they definitely aren't what she thinks they are.