@Willow I sat on this for a while! To be honest, I'm still piecing it all together. My memory is strange (maybe an aphantasia thing).
Mum says, in hindsight, there may have been a few signs when I was very young, but I don’t remember those. My earliest memories which stand out were probably when I was around 10. I think I was fine in the day, but at night when I was left to my own thoughts all I felt was a deep wrongness, and I often cried myself to sleep.
Fast-forwarding a bit, I remember taking a day off school sick, and when I was alone in the house trying a few things on. At the time it just seemed like curiosity. Around then I started to wonder if I was gay, but I'd get caught in a loop of “Am I gay?” -> “No, I can't be because I like girls!”. I tested that out a bit, including a bit of exploratory flirting, but that didn't work for me.
Around when I started my undergrad degree I was reading transformation stories and imagining that for myself. In my head it was a kink thing (somehow, even though it wasn't a sex thing). By the end of my undergrad I was having dreams of being a woman (often pregnant), and grew my hair out. I remember queuing at an ATM and when I turned to speak with my friend a guy in the couple behind us quietly said to his partner “It's a guy!”. I loved thinking I'd been mistaken for a girl but felt a bit gross about being so obviously a guy when my face was seen…