Structural problems in the perception of the self and of the self in interaction with the other are the basis of personality disorders. In Otto Kernberg's modern object relations theory and transference-focused psychotherapy (TFP), this basis is conceptualized as identity diffusion from a perspective partly formulated by Melanie Klein. Her work, however, has given rise to much controversy, which has remained fixed and unresolved within TFP, resulting in conceptual problems. The most important proposal for solving these problems is a shift of emphasis from aggression to the vulnerability of the self. A healthy libidinous connection between child and primary object leads to a meaningful self and a feeling of obviousness of being, while a disconnection leads to borderline pathology, characterized by epistemic yearning and annihilation anxiety, which are mutually exclusive, with a devastating effect on self and identity. The aggression that drives the process of differentiation and integration as a separating force ceases to serve the healthy disconnection of the separation-individuation process but instead serves the pathological disconnectedness of developmental arrest. The result of this paper is a more discriminating reformulation of identity diffusion, thus extending Klein's legacy slightly further and, with it, TFP.