Hi exploradora and welcome back
Oh wow... a therapist dying would be difficult for anyone, to say nothing of a pwBPD. I'm so sorry that happened to him (and you); it's a big loss.
How long ago did that happen?
And then for you to lose your grandfather, with everything else going on... that's so much to handle.
The two of you are under immense stress right now. It's no wonder that his needs (pumped up by BPD) are too much for you at the moment, and your very normal needs (grieving a family loss) are too much for him.
I think I am reading correctly that the two of you are living together in the new apartment? Is that right?
Do you have anyone you could stay with temporarily (family, friends, neighbors)? Trying to think of ways where you can get a break, without having to make a big "divorce or not" decision in the midst of other stresses.
I've heard it said that one shouldn't make big life decisions (divorce, etc) under stress.
I wonder if you can give yourself permission to postpone that decision for the moment -- gift yourself space to grieve your losses, maybe temporarily stay with family or a friend, and get back closer to an emotional baseline, then reassess.
It may be that when you're more grounded, you are OK with staying... or, you may have the insight that you need to separate. However, you'd be making that decision from a place of more calm and peace, instead of in the whirlwind of so much pain.
I really feel for you both, and I hope some of those ideas can be helpful for you.