Welcome to this group- I think you will learn a lot here and feel supported. Congratulations on your engagement.
Reading your post reminds me of myself at your age (reference- I am middle age, BPD mother is elderly). Money is one way BPD mother controls people. College was my path to financial independence from my parents as it meant having space from BPD mother's control.
BPD mother sees me as someone to meet her needs but our task as parents is to raise our kids to be self sufficient adults and have their own lives. Of course we want our kids to still be a part of our lives as too, but we have to acknowlege that they are adults, we don't control them.
PwBPD have difficulty with their own uncomfortable emotions. It wasn't easy for me to see my kids "leave the nest"- of course I missed them being at home. But this is exactly what I raised them to do- and if I missed them, it's not their responsibility to come back and make me feel better. On the other hand, there's also a happiness to see them achieve this- so it's a mix of feelings. But their purpose isn't to manage my feelings.
Karpman triangle dynamics helped me to understand the roles in our family.
https://www.bpdfamily.com/content/karpman-drama-triangleBPD mother is in victim position. Our roles were to do things to make her feel better (rescuer) and if we didn't do that or did something that she was upset about, we were the persecutor.
My BPD mother does the "silent treatment" too- stop speaking to people if she's angry at them but then also decides to speak to them after a while. There's a push-pull pattern to relationships with someone with BPD. Also the rallying family members to "her side"- this is triangulation. My mother has done this too and it's difficult.
I hope through this board and with counseling ( yes, I recommend it to help you to navigate this kind of relationship) you will come to a place where you can be less emotionally reactive to your mother, have some kind of relationship of your choosing (I don't think you want to have no contact with her at this time but you can also choose that if you felt it was needed) and have boundaries so you can have your own independence.
Of course my home is open to my kids but they want their own places to live and I want that for them too. You are not doing anything wrong by wanting your own home with your fiance. You are doing exactly what parents hope grown children will do.