Splitting
At the beginning, in childhood, there is a relationship between
the child and the mother, the parents, the environment. When the
relationship is difficult or painful, the child deals with it
by splitting the difficult from the easy, the love from the hatred.
But to do that, you have to do it with your mind, because it is
not real. You have to split your perception. You have to split
your mind. You have to believe something that is not there. That
is the beginning of mental structure. You have to split the reality
into this and that, split mother into good mother and bad mother.
Well, your mother is never all good or all bad. She is a mixture.
So if you split her into good mother and bad mother, and you have
to remember this and that, you are creating something in your
mind that is not really there. In time, that becomes the mental
relationship that you re-enact in your life relationships. So
there is the idealized mother, there is a frustrating mother,
and there is the attacking mother. And your relationships with
those three parts are what become re-enacted in your life as mental
relationships. (Diamond Heart Book 4, pg 207)

Splititng as Defense
The first defensive operations are those of splitting and projection.
The negative merging is split off, seen as separate and unrelated
to the positive merging, and projected outside; thus it is perceived
as part of the environment. (The Pearl Beyond Price)

The defense of splitting entails splitting away one's Power,
because it is associated with the all-bad self-representation,
and projecting it outside. The result is identification with
a self-representation that is all-good but powerless. (The Pearl
Beyond Price)

Splitting & Narcissism
The idealization of “special” others is a specific
trait of central narcissism, as is grandiosity. In contrast,
in oral narcissism these characteristics tend to be vague and
mixed with various borderline defenses, such as splitting and
projective identification. (The Point of Existence)