Psychiatrists are medication experts and not diagnostic experts. Their role is to prescribe medications and monitor them.

Borderline or any diagnosis is simply a summary to describe difficulties and symptoms. As you can imagine, a summary can be like a snapshot: it can describe you at that moment but it may not be a valid description of you at every moment.
What matters is working with a therapist to learn about yourself, the root causes of the symptoms so you can alleviate them.