Firstly, congratulations on your engagement. I understand how difficult it can be to plan a wedding when a parent has BPD and can see why the situation is stressful. When a child gets married, a parent with BPD can feel it as abandonment and loss of their identity. That can lead to an escalation of some acting-out behaviors.
Everybody wants a mom, and it's not wrong to want to be able to call your mother. I do see your fiance's point, too. BPD can cause moods to change rapidly, and so your mother is less able to consistently meet your needs for love and nurturing. Her mood the next time you ring might not be the same as it was when you last spoke. It is wise to stay centered and be prepared.
I think my mother sincerely means it when she says she loves me or that she is sorry. Her feelings are very real, and I don't think she is calculating or manipulative on purpose. Her behavior does have a manipulative effect, though. She has very intense emotions and doesn't know how to soothe them on her own, and so she will do whatever she knows how to in order to feel better. Sometimes this means begging me to forgive her, sometimes it means yelling at me that I'm the most selfish, hurtful person she's ever known. Her sense of self isn't stable, and so neither can her sense of other people be. It sounds like your mother may be much the same.
its makes me sad, and at the end of it all- I really do hate mental illness. Its a very challenging and difficult thing to watch, live and be affected by so deeply... .
I agree, mental illness is a difficult thing to grapple with, and I feel sad about it too. It helps knowing I'm not alone and there are others willing to support me as I work through it all. I'm glad you've decided to start posting again.
Wishing you peace,
PF