Thank you for replying, Bizawit (you don't have to @ me btw, I'm on the thread). Sex is observed based on external genitalia. Penis = boy, vagina/vulva = girl. Only in a tiny number of cases of DSD may this not be so straightforward and since organisations representing intersex people have asked not to be conflated with transpeople, we can leave them out of further consideration of this topic.
How does gender identity express itself? That’s a really complex question and I don’t think anyone has all the answers. What we do know is that it cannot be reduced in any simple way to what we wear, what we do, how we behave, or, indeed, the physical manifestation of one or more parts of our body (although all of these things may play a part). One important way that gender identity expresses itself is through our deeply held, internal sense of ourselves as man/ woman/ boy/ girl: as you say you know you are a woman.
Quite a lot to unpack here. Starting at the bottom up - I have no internal sense of myself as a woman. I am a woman. I have a female body, I have been socialised as female. I have an internal sense of myself as "Tit" the individual but I have no knowledge whether that is inherently more female/feminine than my mother's sense of herself or my best friend's or any woman on this thread. Or indeed more inherently male/masculine than my father's, my husband's or the man who works in the Post Office's sense of themselves as male. How can anyone know whether their internal sense of themselves is 'male' or 'female'? If not gender stereotypes, what are they measuring it against? What are the markers for feeling 'male' or feeling 'female'?
You say that gender stereotypes are not it but that they play a part. Those stereotypes are a set of expectations placed on us by society based on our respective sex. If we feel out of step with those expectations then it should be society we are looking to change, not our behaviour and definitely not our otherwise-healthy bodies just to fit more neatly within such rigid and narrow stereotypes. "Gender" as a concept - or gender stereotypes, at any rate - only exist if we as a society collectively allow them to. Once we get enough traction behind the idea that both girls and boys can wear blue and/or pink, play with dolls and/or trains, and they can grow up to like shopping and/or sport, be nurses or firefighters, can go back to work after starting a family or be the primary caregiver, cry at sad films or keep a stiff upper lip, and so on and so on throughout all levels of society, the concept of gender becomes meaningless. Why would it matter so much to be seen as male or female, if you could behave and look however you wanted, work and play at whatever you wanted without teasing, judgement or censure? Without "that's not very ladylike" and "real men don't cry" and bossy woman/assertive man conditioning?
To reject being male (or female), you have to be able to define what you think it means to be male (or female), otherwise what are you rejecting? To claim you are female when you were born male (or vice versa), you have to be able to define what you think it means to be female (or male) otherwise what are you embracing? And that's the bit that no one seems to be able to answer beyond those aforementioned stereotypes or "well, you just have a sense don't you?"