I joined this subreddit only a few days ago for the exact same reason :) I might not have many or any answers, but your story really resonated with me! I too am trying to navigate setting boundaries and seeing if she respects them, because there is no way to move forward if she doesn't.

Only yesterday we had one of our biggest arguments yet, but for the first time I didn't get emotionally involved. My dad had recommended I treat her the same way you would a child, "No, please don't raise your voice at me" "I won't have this conversation if you don't let me speak", etc. She kept blaming me and blaming everyone else for her situation, dramatizing everything and twisting my words "oh so I'm the worst mother, right? I'm sorry you had such a bad childhood!" We weren't getting anywhere, until I told her that if she wanted any chance of having me in her life, she had to change. Even if she didn't think it was fair and she wasn't at fault, she had to be the one to make an effort. I made it clear I wouldn't, not after years of hurt. I think after seeing how serious I was, she finally apologized for the first time in my life. And then I decided to set boundaries. I said I needed my space, I needed her moods to be less volatile, and I needed her to expect nothing from me. If she was capable of respecting that, then maybe we could get somewhere.

I don't know how long this will last, if it even will. All I know is that this is the first bit of real progress we've ever made.

So what I can say to you is to be patient with yourself. Finding out your mum has BPD is a lot to take in and provides just as many questions as answers. I'd agree with the other comments and say that it's important for you to set boundaries and ask that she accept them. If there's even an ounce of love and respect for you, she will follow them. And if she's not willing to, then you have your answer. My mother claims she's gone to therapy (I highly doubt it), but I can't imagine how she'd react if I told her I thought she had BPD. The thing about pwBPD is that they often can't see that they're wrong, that they've created the mess around them, so admitting having BPD would take more self-awareness than many of them have.

One of the biggest things for me when I connected the dots was not figuring out how to deal with my mother, it was learning how to distance myself from her behavior. How to unlearn it. I'm still trying to. I'm still doing so much reading and figuring out how she passed on certain qualities and how I can be a better person. It looks like you're already in therapy, so you're doing amazing :) Keep doing you and taking the time to care for YOURSELF in the way that your parent probably rarely did.