Hi ADV and welcome to the board.
It is great that your mom is in therapy and you want to support her. It is a tough road for the both of you and for you it is important to have a support system as well. We can help with that part.
The best advice I have is to learn about some of the defenses that pwBPD (people with BPD) tend to use when dysregulated. Understanding what is going on helps to depersonalize the behaviors so that we can respond more appropriately rather than reacting. The defenses used, like projection for example, are things all people do at some time or another but they happen more frequently in pwBPD.
The other thing is to learn communication tools and boundaries. We have lots of things I can refer you to to read but it is *a lot* of information. Self care is going to be vital for you. Taking breaks, having a support system and even using the tools. The tools don't just help your mom they will aid you too by helping to keep things calmer.
If you had to focus on just one challenge you have with your mom what would it be? What is the biggest one? Lets work from there.
Again, welcome.