With any kind of mental health, the person “suffering” from it has to be the one who wants treatment, and with BPD, most of the suffering ends up outsourced to others, which eliminates that motivation.
I know this is hard to hear, but part of BPD is an inability to form healthy relationships. The flip side is that it takes two people to maintain an unhealthy relationship. You’re not helping her by staying in this marriage; at best, you’re enabling her and at worst, you’re suffering from her mental illness at least as much as she is.