Didn't realize this when I wrote original post, but after a search I have discovered there is such an syndrome called "White Knight". Looking into this more... . the more I study, research and learn about BPD, the more I discover about myself and that I've got "junk" in my own trunk. And that we all, to one degree or another, have a little junk in the trunk.
I almost responded to this last night to let you know there was such a thing and that only you would know if it fits you.
I'm glad to see you researched it on your own.
Yes, we all have junk in our trunk. It's often what links us to our mates. There is nothing wrong with wanting to help people out. The only problem is when you end up with people who might say they want help, but don't do anything to help themselves and you allow them take you down with them. You don't want that to happen, and it can with pwBPD if you aren't careful.
You don't know what your finace's failed marriages were like. It's possible they were just bad matches. It's possible all the problems were caused by your fiance. It's possible she chose poorly each time and each man was really a cad. It's possible that you might be the next failed marriage no matter how much you care, love, help, etc. You can't base how you plan your life on thinking that you can change her life. You might be able to do so. But what you have is what you have. You have to be prepared to deal with what you have, assume she will not change and know that the rest of your life with be helping her manage her BPD. If she does improve, that will be wonderful. But you cannot assume it will happen, as then you will feel like a failure if she never improves, or only gets worse (as is wont to happen) until you can set up proper boundaries for yourself.
PWBPD can be incredibly funny, intelligent, caring, wondrous people. It can be a (mostly) good relationship if the spouse (and/or family) know their own boundaries and learn how to validate properly to diffuse the situation or recognize when it's time to get out of dodge for a while. I'm getting better at that now that I have found this site. But I have no delusions that H will ever be a person that I won't have to watch my words with or can handle stressful situations without some BPD response flaring up. I accept that and don't expect it to change.
I think someone else said it: this isn't a sprint. This is a marathon that never ends.