Hi SeekerofTruth!
Before I adress your questions in more detail, I would like to ask you if you are diagnosed with any cluster B disorders, or if that is something you believe is true about yourself?
In general terms there have been some suggestions in literature about BPD and NPD that actually as a combination there can turn up a dynamic that sort of, (in a dysfunctional way, but depending on where you are standing also can seem functional), makes such a r/s sustainable... .
One of the people I have seen writing about it is Sam Vaknin, (however a somewhat controversial writer, since he is not educated in psychology but something entirely different but claims to be a diagnosed NPD himself and writes about it). He has stated that a borderline person due to their unstable mind and problems in sustaining feelings becomes more of a challenge to a pw NPD, since those factors keep the narcissist in what he calls the hunting mode for longer periods of time. (My guess is what he calls the hunting mode is what here more often is referred to as the idealization phase... .). And through that the narcissistic party doesn't loose interest in the partner as fast... .
On the other hand the borderline party is less sensitive to the narcissists selfishness, since the borderline usually have a fair amount of that him or herself... .And as long as the borderline feels the narcissist is hunting for them, they feel validated... .
Now all of that is just theory here... .And there is no telling if there is any truth to this whatsoever!
So back to you! What are the problems you are having in your r/s?
What is it you feel you are loosing?
You write that you feel helplessness... .In what way?
And what would be the wished for redemption that you are seeking?
I know it takes a lot of courage to delve into this personal inventory when dealing with the dynamics of a relationship - so kudos to you for doing it!

Best wishes
Scout99