An increasing number of studies are investigating the links between autism and borderline personality disorder. Studies report overlapping differences and the challenges in differentiating between these two diagnostic labels. In practice, there are many people, especially autistic women, who feel that they were misdiagnosed with borderline personality disorder. This study aimed to explore the experiences of autistic adults who were previously diagnosed with borderline personality disorder. This is an interpretive phenomenological study. Data were collected using one-to-one, semi-structured interviews. Interview audio-recordings were transcribed and analysed using an interpretive phenomenological analysis. Ten people participated. All recalled autistic differences since childhood that went unnoticed. In most cases, borderline personality disorder was felt to have been a misdiagnosis. This misdiagnosis carried stigma, introduced diagnostic overshadowing and led to harmful experiences for our participants. While they did not identify with the diagnosis, they felt powerless to challenge it. In contrast, receiving an autism diagnosis was 'life changing'. While this did not solve everything for them, it was deeply validating. It also allowed them to be their true authentic selves, shifting the focus away from 'treatment' and changing who they are to suit others, towards acceptance of their differences and neurotype. This significantly improved their mental health.