“Splitting” is a term that describes difficulty with the ability to hold opposing thoughts, feelings, or beliefs about oneself or others. In other words, positive and negative attributes of a person are not joined together into a cohesive set of beliefs. You view yourself and others in terms of good or bad and there is no in-between.
Splitting is very common in people with borderline, and it leads people with BPD to view others and themselves in 
“all or nothing” terms. They may see someone as perfect one minute but shift to seeing themselves as all “bad” or even evil the next. This makes it very difficult for someone with BPD to form lasting, healthy relationships.
The borderline uses primitive defense mechanisms which individuals without BPD outgrow during normal development. People with BPD are ruled by emotions and find it difficult to put feelings aside and allow themselves to make rational choices.
borderline personality disorder 