I remind myself of the symtoms that, in hindsight, made me think I might be trans in the first place.
The need to cross-dress, and the relief I felt when I did, were not imaginary. Those things were real. Wanting to be a girl when I was little was real. Being repulsed by men and male culture was real. Wanting to do stereotypically-feminine things (baking and knitting) was real.
And now, I can add the things that have happened since I started transition. The quieting of disturbing mental chatter since I started HRT. (I won't describe the offending imagery, as it could be triggering for some, but I am really glad it is gone.) The sense of joy and liberation I feel when I am myself in public.
These things are all real. They are not something I have invented in a moment of craziness. When I have doubts, I remind myself that I would have to pack away all of them and pretend that they were not real if I were to give in to the doubts. I lived that way for 60+ years, and if made me miserable.
The doubt doesn't last long.