There is literally nothing you can do to prevent being devalued by a narcissist and I will tell you why. You have done nothing wrong but believe the narcissist and that is not your fault either. Narcissists do not meet you and say “hey, i am going to manipulate and brainwash you”. Narcissists discard what they deem “objects”, not human beings. They discard because they are obsessed with controlling and receiving supply. Lets say there was no reason a narcissist could pick a fight with you, the narcissist will devalue you for sneezing too many times. There is literally nothing that can be done to avoid this. The main trick of a narcissist is to pick the best opportunities to begin this devaluing and those opportunities are always the times you expressed yourself in any way, this way you will inevitably ask the question you just asked here. Blaming yourself for a narcissist’s malignancy is common but never forget, it is a projection.