Diagnoses are voodoo. It's a best guess, a direction. Nothing written in stone.
You mention going back to treatment again. May I ask what's sent you back?

Personality disorders are kind of like normal human impulses and desires except blown sky high. People can have PD traits, yet not really "qualify" for a full blown PD. If your previous treatment benefited you, then it's possible you aren't displaying very strong BPD traits anymore. From your post, it sounds like some depression in the forefront, and that can cloud almost anything.

My doctors don't really work from my diagnosis. We treat my symptoms to bring them under control and we explore my thought, belief and behavior patterns to identify and adjust the unhealthy ones. I haven't had any particular "BPD-specific" therapy. We treat it more as a system-wide thing. Perhaps you can work that idea with your doctor? Is there a "right" treatment? I think anything that can help is the right one for now!