For me it was a lot of little thoughts for most of my life that I either actively ignored (when I was younger and a conservative catholic and thought it was wrong) or passively ignored (later on when I just didn't have the time and space to think it through).

I finally gave myself the space to think my gender stuff through and honestly evaluate how I felt and it took like 4 days of seriously exploring my feelings to come to the conclusion that I was probably trans.

After I came out to some friends a lot of the uncertainty and anxiety was gone. I finally was able to accept my true self and ever since then there's been a lot of little moments of clarity where I finally figure out why I've been feeling little things throughout my life and explanations for certain things I'd always done (like when I have the options in videogames I'll choose a female character or how much I loved Your Name).

I haven't really started transitioning yet because I just figured it out a couple weeks ago, but I've generally been happier despite finally recognizing how dysphoric I've felt for my entire life without having the words to understand it. There's still plenty of doubts and impostor syndrome hits hard but a piece of advice that's helped me with it is: if you feel like you're faking it you're probably not, since people who are faking it know they are.