Hi Finallyawake,
I decided that this was going to escalate out of control if I stayed in the conversation. So, I said that I have apologized and admitted I was wrong. If she chooses to not accept my apology, that is fine, but I am not going to continue the conversation any further. I walked out of the room and did not come back.
This morning, Mom acted as if nothing ever happened. She said, “I thought we were going to spend time together last night. Do you think we can spend time together tonight?” I said yes. She never addressed WHY we didn't spend time together last night.
Should I have mentioned why, or does she secretly understand that I was not going to put up with her behavior last night?
I find that when loved ones with BPD start behaving in a manner that is unacceptable, walking away is a great solution. The thing is, I wonder if this sort of takes advantage of their fear of abandonment. I noticed that when I started doing that with my own uBPD mother, she upped the ante for awhile; I guess they call it an extinction burst.
In any case, I don't know how conscious your mother's behavior was. Your walking away from her could have been so upsetting to her that she blotted it out of her memory. Maybe her will behave better to avoid being left behind. Or maybe the next time you start to walk away from her, she will find a way to escalate the situation to prevent you. She will either accept your boundary or fight it. I recommend you maintain your new boundary.
Also, if she does bring it up, do I simply restate that I am not going to participate in a conversation that makes me feel uncomfortable?
I don't think you need to talk about. Speak with your actions, not words. Also I doubt she will bring it up except to accuse you behaving badly.  :)on't J.A.D.E.  :)on't Justify.  :)on't Argue.  :)on't Defend.  :)on't Explain.
What if she accuses me of upsetting her or making her sick by walking out of the room? I don't think she will say either of those things this time, but I have seen her say stuff like that in the past.
Give it time. She might just do those things again. Just re-apply the boundary. "I'm sorry you felt upset or sick because I walked out of the room. Period." If she escalates, retreat. I would.
I talked to her today on the phone while I was at work. She sounded totally 100% okay and not mad at me at all.
If she does decide to "punish" you, I think it would be for something completely unrelated (at least at face value). Keep an eye out for that.
Taking care of my own needs and actually consciously managing my relationship with my mom is so new for me. Any advice would be very helpful. Thanks.
I have found that the best allies for maintaining low contact with BPD loved ones is: (1) formality and (2) distance.
Best wishes,
Schwing