Hi bus boy,
I'm sorry that you're going through this. As I was reading your post, it resembles a persistent pattern with my ex.
S9 was all good and dandy with me picking him up today but she had to put a I'm hurting our son for my own selfish needs twist on it.
My ex will often triangulate and rescue the kids and cast me as the persecutor. It took awhile to see the pattern on a consistent basis and she always blames me but what helped me, it may help you, is that I quickly recognize the drama triangle. I stay out of the drama triangle by not choosing sides and remaining in the middle so that it stops the vicious cycle of blame.
Your ex says that you're hurting your son ( rescuing him ) and casting you as persecutor, BPD is a persecution complex and the person's circumstances are almost always caused externally by someone else which is not accurate because we're responsible for own actions / choices. Obviously sometimes some things are not in our control but it's not almost always caused by someone else. I suggest to read up on our drama triangle article and you'll probably see a pattern with your ex, it should help with depersonalizing the behavior, and stop the vicious cycle of blame. I hope that helps.