There are three key pieces of information that narcissists frequently collect in the idealisation phase of the relationship that they later wield against you in the devaluation and discard phases using their language of cruelty:
1) The flaws, shortcomings, insecurities and secrets you’ve confided to the narcissist
2) Your strengths and accomplishments, especially the ones of which they are pathologically envious
3) Your need to please them and their need to be perpetually dissatisfied
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The Secret Language of Narcissists, Sociopaths and Psychopaths: How Abusers Manipulate and Traumatize Their Victims