"I always knew on some level"
This is a comment you'll hear many trans people make. The problem is that this comment does not mean what you think it means as someone who is not transgender.
A person who's not transgender does not have to think about their gender. It aligns with their body's sex. Their preferred mode of expression also tends to line up with societal norms for their sex. (By "sex", I mean what the doctor decided their sex was by looking at them for 30 seconds or so at birth, and scribbling something on their birth certificate. This sometimes has nothing to do with their biological sex at all. But ignore that for the time being.)
Your parents do not ask you "are you a boy, or are you a girl?" You are told this - and you know what? For the most part, it just kind of works for you. Yay! If you happen also to be attracted to the opposite sex, then jackpot - you don't have to really think hard about your gender, or who you are sexually attracted to.
It can all be boiled down to a quick examination of your genitals - "yep, girl parts. I'm a girl. I like people with boy parts!" (BTW, I'm really convinced most of you really do think in terms like this - I spend more time talking about genitals with people who aren't trans than I ever do with people who are trans. I think many of you really do see things just about that simply!)
So you know your gender - not on "some level" but on pretty much every level! Your mind matches your body, and if you are really lucky, your sexual orientation! Life is good!
OK, for someone like me, who's transgender, it is not like that at all.
Now it's true, some of us - not that many but some - identify so strongly as our gender that we assert from a very young age "I'm a girl!!!" despite having boy parts. This isn't common, and even some of us who do feel that way are often really beaten down and forced to conform. Because most mom's and dad's, until very recently, don't take such assertions well, and many even to this day, still do not.
So for the rest of us - we feel "something." Oh this usually starts at an early age, and it really gets going good for some of us as we hit puberty, and our bodies start to change in ways our brains rebel against. This is a weird feeling - seeing your face and body change in ways that repulse you, even as others tell you "my goodness - you are a handsome boy!" But we don't feel handsome. We feel horrendous!
This is extremely confusing - all the more so because if we do start to act out in some way, such as crossdressing, we've historically been told that there is something very wrong with us. Literally everyone in our lives, and everything we read, see on TV, etc. tells us "people with boy parts are boys, and people with girl parts our girls." It is very difficult to tell people "but I'm not a boy!" when everyone can look at you, and see that to their eyes, and their way of thinking, you obviously are. Indeed, for a very long time, we were thought to be mentally ill. Life is tough when your internal reality doesn't match what everyone else sees.
And until very recently, with the advent of the internet, most of us had no way to find others like us. (Indeed, I didn't meet another transgender person until age 50.)
As kids, nobody handed us "The Little Golden Book of Gender" that lead us to realize "aha! I'm really a girl!" The message most of us got, instead is "aha! I'm really some type of freak!" And so like most people who are told they are doing something wrong, many of us chose to hide these feelings as best we could - preferably even from ourselves.
So please, when you hear one of us say "I always knew on some level", please don't equate that statement with the certainty that you, as a non-transgender person, has always felt about your own gender identity.
I hope this helps clarify the issue.