I don't know if I can be of much help, because I feel a lot like you do. I have been married to by wife with BPD for 13 years and felt like this for most of it. I can't find a way to improve the situation and obviously haven't found a way out yet.
Know that you're not alone and there must be something that attracted you to her in the first place; somehow, someway there's a good person under all the crap. We have to keep revealing that person and encourage her.
Thanks, I'm sorry you feel this way too. The thought of 13 years makes me feel like im heading for an early grave. I'm torn between thinking there's a good person underneath and actually this is all part of the person -good and bad.