Narcissistic infidelity differs from "normal" cheating, because a narcissist feels no shame or remorse for what they've done. In fact, they convince themselves it's your fault, and actually lead the new partner to believe they're a victim of you. A narcissist will talk about marriage and having kids with you, while sleeping with another person. They'll give you the silent treatment and punish you, trying to make you feel bad for their own bad behavior.
Survivors often wonder "why wasn't I good enough?" or "why is the new partner better than me?" because the narcissist will shamelessly wave this person in your face and parade the new target around on social media. Every second you spend comparing yourself to this person will erode your self-worth and fill you with feelings of inadequacy and rejection.
How did they replace you so quickly, immediately making all the same promises to another person? The answer is simple: Cluster-B disorders all stem from the inability to attach. They never attached to you, which is why they try to intensely manufacture all the normal feelings of love and bonding, and it's also why they are able to detach and do the same thing to someone else in one day. Because they never successfully attached to you, despite all of their sweeping words.
Sociopaths and narcissists are incapable of attaching to other human beings, so they hone all of these other skills like seduction, flattery, mirroring—all in an attempt to mimic what they see other people doing: loving each other. The problem is, they see "love" as receiving constant attention and adoration. This is what they give to you, and this is what they want to receive in return.
The NY Times describes it this way. "Narcissistic alexithymia: The inability to understand or describe the emotions in the self. Unable to know themselves, sufferers are unable to understand, relate or attach to others. To prove their own existence, they hunger for endless attention from outside."
Narcissistic "supply" is really just a distraction from this condition. When you fail to relieve this (because no external factor can), you are punished and replaced. No matter how caring and kind you were, they still don't feel good, and their disorder convinces them that a new partner will be the magical fix to everything. This is when you get "split" as the crazy bad person so they can justify their sudden change of heart.
Even though they blame you, I hope you can see that this process has literally nothing to do with you. You can follow their new relationship, hope it fails, analyze yourself, analyze them, try to be more perfect, prove yourself, figure out whose fault it was, etc. All you're doing is hurting yourself.
Turn your focus from external to internal. What do you feel? Inadequacy? Shame? Rejection? Betrayal? These are your feelings, and those are what matter. You need to work with these feelings, understand them, and learn to offer yourself the comfort and love needed to heal them. Otherwise you're just left with an unresolved mess of pain from an impossible situation, and a frightened heart that believes it's at fault. Left untended, it'll eventually just fade into a numb obscurity.
Work with this pain, understand it, talk with it, communicate with your body. This is the most important thing you can do. Every time you're tempted to check on them, ask yourself what you're feeling. A void? Emptiness? Loneliness? Resentment? Numbness? These issues cannot be resolved through searching externally. Yes, they were created by an external factor, but it's your wound now, whether you want it or not. You are the only person who can decide to nurture these wounds and build a loving relationship with your emotions.
Infidelity causes so much jealousy and humiliation. This can set us on a path of revenge or anger, which is quite normal. We see it all the time in the recovery world: "HAHA HIS NEW LOVER IS SO AWFUL, I'M BETTER THAN HER! HE IS UGLY AND PATHETIC! I'M GOING TO EXPOSE HIM TO THE WORLD!" but this type of manic anger keeps us from feeling the painful, unbearable emotions that are completely natural when we encounter such a drastic rejection from a trusted loved one.
In the end, the most important thing we can do is find the wounded heart underneath all of that, and recover our own ability to love and attach.
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