zachira 
When a person with BPD drops their facade of niceness [...]
Yes, makes sense. This seems consistent with that pwBPD's behave worst with people who are closest to them. I think there's a lot of causes and reasons behind this aspect of the disorder, and I don't think they have this deliberate and conscious chain of logic where they "drop" their act. I think it's quite automatic/learned, which emphasises your point that you're not responsible for this aspect of their behaviour—it's not about you, as you put it.
It also helps me to realize that boundaries are key.
Yes, I agree boundaries are critical. Boundaries are a lot about maintaining the dynamics of a given two-person relationship and how you
want it to go. Not how you may have been wishing it to go. With "normal" people they seem to have this automatic set of structures set up—but with a BP, particularly in a close relationship, the sort-of fluid structures of how they operate (socially and psychologically) mean that you have to be the one to set up those boundaries and structures in your relationship with them.
I am realizing it is important to meet people where they are, and not where I wish they were.
Yes, agreed.
I'm glad to see you're somewhat enjoying your peace.
