I agree with Kathy that it would be good to have your mother speak to a professional...it's kind of necessary so an impartial third party is providing facts to her. It might be a good idea to have your mother see a different therapist from the one you are seeing. All therapists should be professional and provide the same "diagnostic" details about gender dysphoria and ->-bleeped-<-, at the same time as not appearing to be someone "on your team" trying to convince her. I know it's complicated, but I ran into this problem, and the only way around was having the other party choose a therapist themselves.
You are at the beginning of a long and difficult, yet fulfilling and necessary journey. I wouldn't turn back now for any reason. I am happier being Kat than I ever was before...and I'm continuing to experience life with those around me like I was always Kat.
You've got this!