Hi Itsrainingapples! Welcome!
Thank you for being brave and posting some of your story here with us! You are not at all alone. So many of us will be able to relate.
I am very glad that you've found such a great T who understands BPD. That is wonderful and such a key to helping you to recovery. Sounds like you are learning some great tools to help. The scary and unsettling part is actually walking in that recovery and taking the necessary steps to care for YOU. It is strange isn't it, yet so empowering!

I have been advised to stay separate from my uBPDm's house too by my T. Here's the strange part: my mom passed away in 2012, yet I cannot easily stay in her house because it is so extremely triggering for me. She is still every place in that house, the decorating the same, and I feel her presence there all these years later. My sister struggles with the same thing.
She will not let my last visit home go. She is prying and digging into why I didn't want to stay at the house, which I guess I expected. I had told her (before going home) that it was to better build our relationship and I was sorry that it made her feel so bad. But that's not good enough for her, she needs more.
To answer your question about the pressure to tell her more, I would recommend that you stay firm and not yield up any more information than you really want to. Emotional enmeshment is a common characteristic in a pwBPD. I was terribly enmeshed with my uBPDm, feeling as if I had to tell her everything. She would pout or give me the silent treatment or FOG if I did not tell her what she insisted on knowing. I still struggle with that as an adult in my 50's, the telling of too much and feeling obligated to do so. I think one of our greatest desires is that our pwBPD would hear us and understand, once and for all, how they've affected us,
but it will not work. I am sorry for the hurt that this causes, and those of us here on the journey to recovery have experienced the disappointment and grief that goes along with it.
There are a couple of great tools we frequently recommend using to communicate with a BPD. I will list one below. You don't have to give reasons why to her. You don't have to defend. With practice it will get easier and the FOG will ease. Be patient and kind to yourself as you journey. It takes time. Click on the link below for a workshop about 'jade.'
Don't JADE Wools