I'm really struggling setting boundaries with my mom who has BPD.
Hi turtle72, welcome and hello. Setting boundaries is so challenging when someone has BPD. Having weak boundaries or none at all is distressing, and to have it be your mother adds additional stress. I'm glad you reached out. It's hard to walk this path alone

I found I did best with small goals. It's also a bit confusing setting boundaries with someone who may have taught you that having no boundaries is normal. How have you asserted boundaries before with her? Details do seem to make a difference in these relationships.
Maybe we can help suggest alternate ways to try things that might work.
Was your stepfather abusive? It sounds like he was not a safe person to have in the home, making things doubly hard.