Forgive me if any of this is unclear, but I am writing this in part to better process my answer for myself. I've "known" this about myself for as long as I can remember, even if I've not always been able to accept it. I don't know if my experience is similar to people who have actually transitioned, so maybe you wouldn't actually call me trans, or if this is in any way what you might call a "typical" experience for anyone in this community. But I'm going to write a few memories, as well as what I felt during them, and hopefully this can be some help for you, too.
My earliest memory about any sense of dysphoria was at age four, when my family was taking me to church one sunday. I would look around at the outfits the girls got to wear with huge feelings of envy. They were pretty, and I admired the way it made the girls look. I also remember intense jealousy, because I was not allowed to express myself by wearing a dress like that, being stuck in "stupid boy clothes". These were thoughts only at this point, because I also already clearly knew that these were Bad Things to talk about, so I'm sure I had at least mentioned something similar to my parents before this, though I don't remember how early that actually was.
At about eight or nine I was first exposed to romantic movies or tv shows, or at least began to pay attention to them when they were on. Every single time, I was almost spellbound by the experience the woman received in these stories, and I knew rather clearly that it was an experience I desired. I should note that at this time, I wasn't really focused on the "man" or the "woman" per se, but more on the roles they were playing in the relationship. I knew that the way she acts, the way she is treated, is something that would bring me joy for myself. At the same time, trying to imagine myself in the male role, I just couldn't see myself acting in that way, nor getting nearly as much pleasure from his role. I never did act on any of this though.
Puberty was the biggest challenge I have faced in my life. I was acutely aware around age 11 that the girls were beginning to change. I experienced the full spectrum of jealousy and anger that I wasn't getting breasts or their shape. I knew for the first time that I really wanted that to be my body, too, and first began to ask myself why I couldn't just accept that girls were different than me. Why can't I just be happy being a guy, since that's what my body is, what family/friends/people tell me I am? I've never been able to answer this question to this day.
When I was twelve, my beard began to grow. At first, I was genuinely proud of myself. My first time shaving, I remember thinking, "Finally, I can be more like Dad." This joy lasted all of a week (the second time I needed to shave). I quickly began to resent shaving. Even though it's just hair, it was the first time it really became "real" to me how different I was from what I wanted. I resigned myself to it, thankful at least that my beard doesn't grow as fast as other guy's, nor as full, so I don't have to shave every day if I don't want to.
There were many nights throughout the rest of my teenage years that I actually cried myself to sleep, or cried through the night without sleeping at all, feeling like my body was "slipping away from me." Every passing month, every passing year, made it more clear, more definitive, just how NOT female I am supposed to be, even though I was so sure it was what I always wanted. By turns I would beg and plead with God, demanding he fix me, or answers for why I was made this way. Then I'd spend a while accusing myself of idiocy and insanity, not understanding why the hell this felt so important to me. Back and forth for years.
By about eighteen, I was finally able to suppress those emotions enough that they wouldn't cripple me as I tried to function during my life. In the ten years since, I've been able to ignore it, mostly, though I suffer from what my therapists have called "chronic disthymia". They've never found the source, though I suspect this is the reason. But it's been so ingrained into me that We Don't Talk About That by my family when I was a kid, that this text is the first time I've ever communicated it to anyone.