My understanding, out of all the personality disorders, this would be one of the toughest to diagnose and treat. Oft there's unwillingness, getting to a diagnosis is difficult.

Since we can't change others, we can change our reactions, internal responses, learn detachment and coping. I've known a couple of people, myself included, that started therapy because of toxicity in relationships, many due to the effects of having loved ones with some disorder or another, and discovered a sense of , for lack of a better word, empowerment over their own lives.
If she is NPD, then it stands to reason it's had a lasting effect on you, and resolving what that means for you, matters.