How can lovers feel both love and hate in the same week or even hour or be devoted for years and then cut off all good memories? This is a manifestation of the defense called splitting, first coined by Freud.
It starts in infancy. To separate from our mother (or earliest caretaker), we must make sense of contradictory feelings of love and anger toward her to develop a cohesive view (“object constancy”) of her and ourselves, meaning that we internalize steady images of our mother and ourselves. When mothering isn’t sufficiently nurturing and consistent, we’re unable to integrate good and bad feelings about her. To cope, we mentally split the good and bad mother into two contrary representations. Splitting keeps the “good” and loved aspects of our mother separate from the “bad” and hated aspects of her.