I fully came to realize the nonbinary thing when I wore a dress to be a bridesmaid in my dad's wedding to my stepmother and i looked in the mirror at myself in the dress and it was this legit body snatchers type feeling. Like i was looking at someone else wearing my skin in that dress my brain going "that is NOT me".

And then i realized ..."this might be dysphoria". At least a mild form.

And thinking on it , i remembered feeling kinda this way about getting dressed up foe church as an older child (10-12?) the reason i hated it was because I'd look in the mirror and feel like it was very much not me when i was dressed like that.
Which also may have been part of other body image issues i had as a teen , and young adult, that were part of why i resented my parents' attempts to instill better hygiene skills and pride in my appearance . I just have a whole complicated thing about feelings towards my looks and body)

(And thats also why most of the time now i just don't DO full-on dresses)

For the longest time I would go months where I just pointedly avoided looking in the mirror.

Anyway sorry for all the tl;dr but writing all this out helps me process it.