When I started to question myself about my gender identity I was seeing a therapist, I talked about this with her but, since the reason of my therapy was other gender identity and we were already working on so many other issues, she said we could explore that later. Sadly she couldn't see me anymore and I couldn't bring up the topic again. My next therapist after her... I never really liked her and when I talked about gender matters to her she almost said to me "no, you're not trans!". I stoped seeing her after a few weeks and decided I would take care of this myself.
I have, I've done some research about what being ftm or transmasculine is, about gender dysphoria and all that. I'm sure now that I am non-binary and I did that with no help of therapy (at least not direct therapy as I read some articles wroten by certified gender therapists).
However, I would like to take therapy later on the future, it's scary since I don't want to face the same expirience with my other therapist and I don't think a lot of people know what non-binary is. One of the reasons why I think my last therapist discarded GID on me was because I said "I don't feel neither man or woman" and she probably had never heard of that before...
I don't think a professional diagnosis is as much as a requariment to be transgender, but for some people, like myself, maybe talking it outloud with a professional helps, if the professioal is actually good knows about the matter, of course!