Apologies for chiming in when I do not have a child with BPD.
I do however have an extensive history of living with/alongside persons with BPD. Mom, sis, partners, friends, etc.
Firstly, sorry you are in such confusing and stressful circumstances. I cannot even begin to imagine the angles and anger coming at you in this dynamic and also having to manage your own feelings in all this mess.
My first/only advice today is to know what is your responsibility and what is not your responsibility.
Simply put... .(but not so simply practiced)
Lesson 1:
Your feelings = your responsibility
Anyone elses feelings = their responsibility
Right now you feel your spirit is being attacked. It very well may be. They very well may be venting anger and all kinds of feelings at you. You even may feel guilty because of things that have happened in ways you wish they hadn't.
However... .
Refer to lesson 1
Reread
Repeat
Attempt to apply to situation
And continue to do this again and again until you feel you have neatly sorted out whos feelings are whos in this mess.
See if anything in your perspective shifts for you
See if you can identify who is responsible for what
(Always returning to rule #1)
Return to us to think some thoughts you are comfy sharing out loud so the community can gather round to help you sort it out some.