Something from the past changed (also in the past): I had never met him before the night he asked me to marry him.
Something from the past is changing now: I have never met him before. I can't wait. (As I stand waiting to meet him.)
Something from the past that cannot be changed now:
I never met my birth mother, and now that she's dead, I never will.
Something from the past can still possibly be changed in the future: I have never met him.