Borderline personality disorder (BPD) is a severe and relatively prevalent psychiatric disorder, responsible for high rates of suicidal behaviors. Disturbed identity appears as at the very core of this disorder, being inter-related with all other BPD features. Notably, from a dimensional perspective on mental disorders, one should realize that it is from our usual self-representation that we live all our daily experiences. Then, if the understanding of self-concept (or identity) is impaired, all the interventions implemented to decrease the self's suffering will subsequently be impaired. The purpose of the present case study was to illustrate the nine identity diffusion categories described by Jørgensen & Bøye (2022) and how the level of identity function can be improved in a third-wave cognitive and behavioral therapy targeting progressive correct self-identification.