Inside the borderline mind there is a very profound split, fragmentation, and in some cases a shattering of the ego due to the narcissistic intrapsychic injury sustained at a very young age as the result of abandonment (actual and/or perceived) that arrests emotional development.
What does every family member, friend, relationship partner, or ex-relationship partner (non borderline) need to know about what goes on inside the borderline mind? Why does understanding the workings of the borderline mind matter to those who are non borderline?
