Most people think that serial killers are psychotic with a psychopath background. Modern medicine also thinks like that. Some of them were cataloged with Paranoid Personality Disorder, Schizoid Personality Disorder or Schizotypal Personality Disorder.

she was clearly psychotic as a manipulative narcissic

borderline

First of all, if you have any indication that you yourself are psychotic then the likelihood of the person you are dealing with is psychotic also. And psychotic personality is obvious, like crying hysterically or displaying other off the wall emotions such as heavy addictions, anger etc. The answer to your question is that first you must determine if you can live with their personality traits. You may get a long with someone well despite their psychotic tendencies. You can handle their psychosis. Otherwise if you are asking how you can bring yourself to accept that the person you are with is crazy just face the facts, realize you are going to come up short dealing with them and it is not worth it. You basically have to reason with yourself. If you are asking what it takes for you

it is part of a personality