The Little Shaman is a spiritual coach & specialist in cluster B personality disorders, with a popular YouTube show and clients worldwide.
Splitting is very common in relationships with narcissistic people and it can be very confusing. This is the defense mechanism that is responsible for the Jekyll and Hyde personality that you see in your narcissistic loved one. It can be extremely confusing and many people don't understand how it happens.
Splitting is what often happens when narcissistic people experience emotional conflict, especially regarding other people. Because their emotional maturity is so stunted, they are unable to unify both positive and negative attributes about the same object - even when this "object" is a person. Most of us understand that really no one is all good or all bad. Even Jeffrey Dahmer had some good qualities. Narcissists cannot emotionally understand this. They can't accept good qualities in a person they see as bad, or bad qualities in a person they see as good. For whatever reason, their ability to combine these two sides together into a cohesive understanding of a person has been damaged and they are unable to do so.
This is why, when a pathologically narcissistic person feels angry at you or hurt by you, they act as if there is nothing good about you. To them at that time, there isn't. They have dissociated from their positive feelings for you, thus disassociating you from your positive attributes. Nothing you ever did was good, you've never cared and you never will. In a very real way, they see two separate representations of you: the good you and the bad you. The fact that you are a person with flaws and different qualities is something they are emotionally unable to understand. There is good and there is bad. There is black and there is white. There is all and there is nothing. There are no grey areas, there is no in-between.
This probably happens because when the narcissist was a small child, their parent was not there for them somehow. Either abusive or absent, perhaps even unintentionally. As discussed in the article Can Narcissism Be Prevented?, emotional neglect can happen unintentionally. Some children may simply be born needing more. Because children need to feel loved and cared for, the child begins to deny the negative aspects of the parents in order to only focus on the positive - so that they can still feel that they are loved. It is a very primitive, immature defense mechanism designed to combat the stress of trying to integrate two contradictory feelings or types of input about the same thing:
Love is good. How can someone who loves me ever hurt me?
Hurt is bad. How can someone who hurt me ever love me?
This creates a stressful situation in a small child. They are not able to understand or resolve this emotional situation. As an adult, we have learned that humans are complex and we are able to appreciate them in many different ways. We understand that all people have both good and bad attributes, that things happen, that people make mistakes. Narcissists have arrested emotional development and they cannot resolve this situation, either. The solution is splitting:
Love is good. Someone who loves me does not hurt me. You did not hurt me. Therefore, you do love me. You are good.
Hurt is bad. Someone who hurts me does not love me. You hurt me. Therefore, you do not love me. You are bad.
Over time, this creates a pattern of being which results in dissociation from the situation. This culminates in a complete inability to unify the negative and positive attributes of the same object. And of course, remember that you are nothing more than a mirror to the narcissist anyway. Whatever they feel is what you will be reflecting back to them - whether that's reality or not. It has nothing to do with you or anything you're actually doing. Just like everything else, splitting is about the narcissistic person trying to regulate and deal with their own feelings. Pathologically narcissistic people live in an endless feedback loop where they simply feed off of their own emotions nonstop and blame everyone else for upsetting them. This is why we say it's no use trying not to upset a narcissist, because the narcissist is already upset. There is no way not to hurt them because they are already in agony all the time.
Splitting is extremely common for narcissistic people. In fact, the narcissistic cycle of idealization and devaluation that everyone is so familiar with is a perfect example of splitting. All good vs all bad. It's ironic because to those of us who don't have this problem, it looks like the narcissist is holding two completely contradictory beliefs at the same time. They aren't. They're holding them one at a time. The false self they project is an example of splitting, too. Narcissists idealize and devalue themselves the exact same way they do to everybody else. People sometimes ask, "How is it possible for them to put themselves on such a pedestal and also hate themselves so much?" Well, they did it to you, didn't they?