If I see him, I plan to be kind, but keep our interactions as brief as possible and not say anything personal. I am not sure what I can expect though (I have thought of everything from stalking to him blowing up at me when he sees me), and any advice or insights is much appreciated. Is anyone else in a similar situation where you have to frequently see your ex? How do you deal with this? Thanks
OK, so I share a baby with my exwBPD. She is a bit more high functioning than yours, because she doesn't pick fights with random strangers, very often. I can only think of one or two times I observed that behavior.
You'll see a lot on this board about setting boundaries and validating. These two things probably are not very appropriate in your case. Your boundary is a desire for minimal contact. You don't really need to validate him, since you don't want him in your life anymore. Validation, is only good when you still need to show respect to the person. I still need to validate in my case, but it seems like in your case, it will set you up for more drama The best advice I can give, is to not JADE.
Do not Justify, Argue, Defend, or Explain any of your statements or actions from the past or present. Some examples are as follows.
JustifyIf he tells you what you did is wrong and wants to know why. "I don't have to justify my actions to you, anymore."
ArgueIf he states something as fact, that you know is untrue or slanted, don't disagree with him. OK Validation might work in this case. "I understand that you feel X about Y, it makes sense." Just leave it at that, don't try to correct him. That will turn into an argument, and he's probably better at arguing than you ever can possibly imagine.
DefendIf he starts telling you how terrible you made him feel when you said something, don't defend yourself. Just say, "I'm sorry you feel that way." Normally this is a copout answer in a relationship, but if you don't really care or agree with him there's no point in trying to come to a consensus and defend your point of view. It'll just get turned around on you, and you'll be out of line.
ExplainIf he wants you to explain why you said/did such a terrible thing, just say, "This is in the past, and I don't need to explain it to you anymore." If you try to explain your point of view, you'll just get accused of gaslighting or worse.
You might live in the same building, but you can always move if the situation gets untenable.