Bringing a third person into the mix is a powerful way of abuse a romantic partner.
Splitting can be unhealthy for relationships. It keeps the “good” and loved aspects of our partner separate from the “bad” and hated aspects of them.
How love or unloved we feel as children deeply affect the formation of our self-esteem and self-acceptance. It shapes how we seek to love and whether we feel part of our lives or more like an outsider.