I was diagnosed with bipolar, then bpd and finally DDNOS, which is basically (in my case) DID without lost time. I HATED being diagnosed with bpd, as I noticed most professionals treated me as an issue and a diagnosis, without focusing on the cause of my issues. For me, any symptoms that fit into the bpd (those that I met) were also a part of the ddnos and complext ptsd, my new diagnoses.
My world has improved immensly since getting the new diagnoses, as I was suddenly treated as a person, instead of as a diagnosis.
My suggestion is that you think less about the diagnosis, and more about you as a person: what symptoms/issues are the most significant for YOU, and what strategies work for you? No matter what label they attach to you, you will still be the same person you always were. Most professionals have a level of bias when it comes to different diagnoses, and each person with the same diagnosis are still individual people. Having a label is a grest starting point, but it doesn't help much farther than that. Any therapies, strategies, meds, etc are all based on you as a person. What experiences you have had, etc.

I would suggest that if you really like the therapist and he is working for you, that is fantastic. But it doesn't mean that his diagnosis is the correct one. It just means that he works well with you.
It's also possible to have more than one diagnosis. People get second opinions from doctors all the time because diagnoses can often be wrong: it is the doctor's opinion based on the information they have the the knowledge and experience they have accumulated. They are not 100% perfect.

Just some food for thought. I would suggest going with your gut feelings on this one, and to keep in mind that labels change fairly regularly, just look at all the new changes is the latest dsm. You, however, will always be the wonderful, interesting and unique person you always were.

Sorry if that's not really what you were looking for.
Best,
IJ