Hi Rain33
What a challenge it seems you have had in trying to navigate your relationship with your mother! I relate in many ways: I initially thought my mother was uBPD but after extensive observation I came to a similar conclusion as you as to NPD. You may know that the 2 conditions can often coexist? I think that's the case with my mum.
You mention your mother can be critical, judgmental, belittling, argumentative, cold, and lacking in empathy. Could you explain some of the things she has done or tell some specific examples of behaviour you have noticed?
What led you to think that she might have BPD or BPD traits?
Obviously it would be necessary for a professional to diagnose your mother but you don't really need to be a psychiatrist to notice dysfunction or difficult traits. What kind of impact are her traits having on you?
Look forward to hearing more of your story
Ziggiddy