hi zoesmom,
welcome
Your mom had your oldest sibling when she was quite young (20 yo?). My mom had my oldest brother when she was in high school. I wonder if this is common with BPD's to have children quite young?
A lot of what you're posting about is pretty normal stuff. Fear your mom will come here and find your post....fear of the repercussions of that.
I can tell you I worried about that a lot at first, but then I realized my Mother is so out of touch with me and self absorbed, she wouldn't even recognize it was me. I mean I can stand in front of her, a few inches from her nose and scream "HEY MOM, It's me B, do you Recognize me?" and she'll tell me some untruth about me. Like she will say "Oh, you're not an Introvert, you're an outgoing person! You love to socialize."
I am totally an introvert. I take the Meyer's Briggs personality test at least once a decade and it's never changed. I'm still an INTJ, same as always!

That our mothers do not recognize us is one of the gifts.
Have you read the book by Scott Peck "Children of the Self Absorbed." This was one of the first books I read after learning my Mom is personality disordered. It's about narcissism but it really fits for my Mom/Dad duo. She takes on my Dad's personality traits a lot. Like she will go along with his religion and politics, and won't have her own opinion about things.
Anyway, welcome here and you are amongst friends here, we get it. Feel free to share more if you're comfortable. I have literally wrote pages about my Mom (here and elsewhere) and I still find things to say about her. In mourning, in grief, in elation, in joy cause I went No Contact again. It's all good.
We won't judge you here
hugs,
b