LalisaI am glad you are reaching out here! I wish I had an answer to your question about how to help your mom. But what I do have is to let you know you are not alone I have gone through similar things. My father is diagnosed Bipolar and schizophrenic/Schizoaffective. He is not diagnosed BPD but after reading some books on it it fits him very well, he is the one that atleast a month ago would admit to possibly being BPD. He had to mull over that for many years to admit it. Four years ago he accused my mom of being BPD aka 'Projecting'. This was as a family our first hint that he had BPD. Even knowing this he isn't willing to get help, he thinks he can do it all on his own and after reading some books "stop walking on eggshells" included, that it is everyone else's job to change their behavior around him. He is not on any Bipolar medications because he couldn't stand feeling "nothing", plus they didn't really help all the BPD traits, which cause him to be really verbally abusive at time.
You mentioned
When times are good, my mother is amazing, we are all so heartbroken
I have felt the same way about my dad, when he is doing well, he is doing VERY well. I love the father he is and can be, but it makes it harder when he isn't that person. For me it is like having separate parents in one person. But it is hard to push aside the bad times during the good sometimes.
My brother died tragically 8 years ago when he was 33 so there is just myself and my Mum now.
I am so sorry, one of my sisters passed away in a car accident 11 years ago this july, she was in her early/mid twenties. It was sudden and she left behind children. It is devastating to lose a sibling, I miss mine every day and feel for you having to feel those emotions as well.
I have always had an intensely close relationship with my mum - so much so that I always felt that she viewed me as her partner, her parent, her best friend and her child
Me and my non-BPD mother also have a very close relationship, it is becoming strained on my end lately though. Because I come to realize the same things you mentioned. She always called me her "best-friend" and I became a lot of her emotional support and a surrogate "spouse" in many emotional ways, even though she was and still is married to my uBPDf. It is a lot of pressure to be so many things for one person that is supposed to be a parent not also need parenting.
You seem to be very aware of your situation, have you become familiar with coping tools for yourself?