I have had major depression and PTSD since I was 13 years old. Additionally, I was recently diagnosed with BPD.
It took my Therapist three years to diagnose me with BPD. Or perhaps she considered it from earlier on and did not tell me.. Who knows.
The reasons for my diagnosis are the following: intense emotions, a very strong fear of abandonment, constant thought of death (not suicide), a pattern of unstable relationships or lack there of; intense mood swings; chronic feelings of emptiness (which is also a symptom of depression); difficulty trusting; inappropriate, intense anger (and sometimes problems controlling the anger)
Similar to some, I had a traumatic childhood that included armed violence, sexual abuse, an alcoholic parent and emotional neglect. And the traumatic life events triggered the fear of violence, abuse, abandonment, and/or adversity during childhood.
I hope my response provided an answer to your question of "How were you diagnosed BPD?"