Welcome OceanSand: I'm so sorry about your situation with your mom. I think as our parents get older, things can get more difficult and our parents become needier and harder to walk away from. Is your dad still in the picture?
My mom can be so great to be with and then so awful the next moment. People in my town think I have the sweetest mother ever. They don't know what I go through.
I think that many of us have some idea of how is might be for you. People with BPD, (pwBPD) tend to save up their bad emotions and release them where they feel most comfortable. That generally means they release havoc on someone in their family. It took reading about the situation a few time for me to rationalize how my uBPD sister could appear as such a wonderful church-going woman to her church friends and then lash out at family. I used to think that people would naturally treat their family better than nonfamily members, but I've learned that isn't the case with people with personality disorders.
I'm glad that you are seeing a psychiatrist and that you have a wonderful family. Has your psychiatrist lead you to any tools to manage your anxiety? Perhaps you have some favorite ways of managing anxiety that you have discovered on your own?
We can't change others, but we can make things better for us by setting boundaries and learning certain communication skills that can make things better for us. It's possible to communicate in a respectful way that can serve to tame your mother to some extent. It won't mean that she will always be happy with things, but if you are consistent in enforcing boundaries, she can learn that you won't be abused (i.e. you will leave her presence, terminate a phone call, won't answer a nasty email etc.).
What are some of the most disturbing things that your mom does?