Welcome to Adult Children of Mentally ill Parents Discussion Forum!!! Dedicated to the validatation and encouragement towards one another in tranquil and perilous times a mist the obscure insanity of a loved one.
I am 38 and my mother has (undiagnosed) BPD. It has been a long way for me to get where I am now - which is reasonably happy and content in my life. If you can find other mother figures to help you, that's great and very helpful. But if you can't find anybody to help you, it doesn't mean you have to deal with it alone.
My experience has been that I need the safe feeling of having "my own therapist" who will listen to me and validate my feelings.
I don't know your mother or her specific way of being, of relating to you and others. But I can tell you that my mother has never ever been able to acknowledge any of my feelings. She has never understood anything. And this will not change.
So it is possible that your mom will never understand your position no matter what you do to explain it to her. Not because you don't explain it right and not because of anything you do or don't do, but because she can't at this point in her life, with her limitations. Now, your mom may be different, everybody is. But that's how it is with mine and it's taken me many years to acknowledge that. I'm still working on it.
But you made it this far, and you have reached out, and that's a sign of immense strength.
Just wait until you're older and the mentally ill parent disinherits you and gives it all to your equally mean sibling! Then you have what I went through. Actually like William Shatner has recently said, death can be the ultimate cleansing! I have been renewed in my father's absence. It took 10-15 years, but now I love life for the first time since I was a kid.
Hope and time...
At 21 you're ahead of the game by starting to understand it all. Good for you!