Four or five years ago, before I asked everyone to call me SJ and subsequently started using they/them with whoever asked if I was changing my pronouns, my version of impostor syndrome wouldn't let me even consider that I could be like these magical people who had sloughed off binary gender and were using "they" as their pronoun. Once I decided to go to law school, though, I just couldn't make myself do it as uncomfortable as I was with myself. I had one of my friends call me SJ and use they/them pronouns for a few months before I asked everyone to call me SJ, to see how it felt.
It worked. Then I asked everyone to call me SJ, and people started using "they" for me verbally, and every time it was like the piece of me that had been clenched so hard could relax a little bit every time I heard it. Now I'm comfortable introducing myself with they/them in many spaces, and even though I feel like an impostor every time, it still feels so much more honest with myself.
Generic messages of "You are seen" really don't work on me; again, impostor syndrome, likes to tell me that those messages don't mean me, that I don't actually matter. But damn, that feeling of not having to hold my breath is so big. I don't even know why it feels like I'm holding my breath when someone assumes my gender, but that's it. And releasing that breath because of the right pronoun just is such a good feeling. But it took practice and it took the risk of asking people to change how they refer to me and seeing if it fit. It feels like all the nonbinary people seem to just know, with serious conviction, that that's who they are. And that can be hard for me, as someone who has to measure themself up against others to know whether they've done the right thing. But practice worked. I may be in the closet at school/work, but now I know that being nonbinary, being gender fluid, it fits me so much better than assuming I was female just because that's the body I was in. And knowing that about myself is so important to me.