Hi bus boy,
That does sound frustrating. As you know from first hand experience, communication with pwBPD can be tricky.
Have you looked at the tools for better communication? Here is a link to some communication tools that may be helpful.
https://bpdfamily.com/message_board/index.php?topic=69272.0These are about managing the conversations more effectively in order to avoid getting into these circular arguments each time you communicate.
I know you may not want to communicate with your ex but since you have a child together, no doubt it will be necessary.
What do you think?
Hang in there!
Mustbeabetterway