Welcome to the family, Struggling Wife!
I'm glad you've found us here, though I'm sorry you need us -- if that makes sense.
This is a supportive community and you'll find that we can understand what you're going through.
BPD can be a bewildering disorder. And, yes, being the lone emotional/psychological caretaker is an exhausting and unsustainable position. How is your self-care? Do you have family and friends (a support system) that you can reach out to and spend time with, whether they know what's going on or not? What about hobbies? Things you enjoy doing? When we're in a situation like yours, "refilling our cup" is of vital importance.
Also, we have a lot of tools and experience we can share with you that may start to make your situation easier to manage. It's not easy and it takes work and patience, but we've had some pretty dire situations turn around.
I hope as you feel up to it, you'll share more. The more we know, the better we're able to help. If you don't mind, I have a couple of questions.
How do you respond when he makes these threats of suicide?
Does he have a history of cutting people off?
Keep posting!