To me, there were a few things that helped me realized I was non-binary.
I knew that I outwardly wanted to appear more masculine, I knew that I hated how high my voice was, I also knew that I would prefer it if my body was a shapeless/genderless. As a kid (7/8), I had this impression that when I grew up I would be "a girl that looked like a girl and a boy" outwardly. I knew nothing about lgbt* at that point.
At 12, I was obsessed with this really odd book. From What I remember, a girl saw this "beautiful boy" that nobody believed existed. When she found him, he refused to see the town saying it wasn't safe. Then he stripped in the middle of the woods and she saw he had no "sexual" features, which he said was why it wasn't safe. Then he just walked away and she never saw him again. I'm about 90% sure the kid was an alien..?

Anyways, this is when it really sunk in for me. I worked at a restaurant where there were lots of customers, and it was 50/50 whether I was a "miss/mam" or "sir" at first glance. Most corrected themselves after I spoke, but there were a few that kept calling me "sir". At first, I loved "sir" because it meant I wasn't being seen as a girl, and I hated "miss/mam". After a few months though, I started to dislike being called "sir" too.
I knew was asexual since 17, and already read parts of the blog NeutroisNonsense. I thought that I might be on the trans* spectrum, but didn't know for sure if I was FTM or non-binary until working that job. I eventually asked myself if I had to live the rest of my life in a female's body with a female name and pronouns, could I survive? The answer was no. But if I had to live in a male's body with a male name and pronouns, I wouldn't be comfortable either.
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Side note: I'm jealous that you can project your voice. I used to love singing as a kid, but could never project. I would talk to my friends in high school and they would look at me like I was a mime, telling me they had no idea what I said. When I stared my job as a group server, I was speaking loudly and customers/co-workers still couldn't hear me. By the end of that job, I was yelling just so my voice would be heard. Co-workers then commented saying that I sounded angry. It was like there was no win.

As a kid I had extreme anxiety though, and was told I didn't talk a lot/spoke quietly. Meh, my voice annoys me.