Therapists can diagnose clients. If you use insurance then try have to give a diagnosis or they won't get paid. Some are reluctant to diagnose someone with borderline or any other personality disorder because they have negative connotations. Was she treating the symptoms but just not labeling them? There is no medication specifically for BPD and therapists will be more likely to suggest certain therapeutic interventions, especially DBT. A GP has very limited knowledge of psych meds so I'm surprised they never referred you to a psychiatrist earlier. I would definitely bring this up with your therapist. Sometimes psychiatrists and therapists disagree on diagnoses and sometimes they just have differing philosophies about what they think is helpful. I think it's important for you to discuss it so you understand what your T was thinking.