Hello Tweedledum, glad you reached out -- welcome to the group

These are difficult situations, where even though your D maybe hasn't said anything explicitly, you have a reasonable concern that if there is more conflict between you two, she will withhold your granddaughter from you. It's devastating to watch children get caught in the middle when BPD is involved -- of course you don't want that.
It can be a tricky needle to thread, as you're balancing your needs and care with wanting to be there for your granddaughter.
I do have just a few questions to learn more about your specific situation:
How old is your granddaughter? What has your relationship with her been like so far?
Does your daughter (and GD) live close to you?
Are either you or your D in counseling right now?
...
I think you're wise to realize that your own health is just as much a priority as anything else right now. If you aren't at your best, how can you help your granddaughter? It's kind of like the "airplane safety" talk -- first, you have to put on your own oxygen mask, and only then should you turn to help others.
What boundaries would you want to put into place, if you could wave a magic wand? And have you checked out our article on
Setting Boundaries and Setting Limits yet? I'd be interested to hear your perspective on it.
Let us know how you've been doing -- we'll be here.
kells76