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Do it
(feddit.org)
For me Its quite simple. My brain decided it. Literally. The days I found out I'm trans, my brain would do this weird thing, where I could think about completely different stuff and suddenly my now chosen name would "fly" into my mental Field of View, like an asteroid in star wars or so. I told that name a close friend and 4 weeks later when we went to the club together I told him, that I will stick with it, because I didnt had a better idea (and it was the only name I really felt comfortable with).
I was lucky enough that my Mom has told me what I would have been named if I had been born a girl. I haven't come out as non-binary to her, but the knowledge is still lovely.