you can dress however you want, girl. you aren't posing by being yourself. a tomboy is still a girl.
i don't identify as woman, but i dress like one anyway. i don't feel too much discomfort in my female body, though i'd like to get it changed to fit me a little better as soon as i can in the future.
gender is an interesting thing, that some times doesn't really fit perfectly with your birth sex, and neither with your image of a body you can feel comfortable in.
it's not too late to realize at 19, the reason why your body feels wrong. suppressing thoughts that don't seem to fit into anything you've ever heard of is almost incredibly easy, and it might even be that you couldn't get all the way to thinking the thought, because how can you consider something you don't know exists? it's a scary thing to suddenly realize you have thoughts that don't fit with what you know of the world and how it's supposed to work. i did know i was trans when i was in middle school, but i completely dismissed the thought because what i thought trans was, or had to do, didn't fit with my own feelings of who i am. took a long time to realize again how trans i am, and how impossible it will become for me to deny this fact.
you might want to consider a breast reduction surgery, rather than the mastectomy that trans men typically get. the smaller they are, the easier they are to bind down. and maybe you won't even feel the same need to hide them if they are small enough. just a though that you might want to examine for a little bit, i know someone non-binry who had a breast reduction because that would allow hir to be woman or man or androgyn as they felt like it. easier to bind, but still looks ok in a bra. it would be a tiny bit weird for a woman to want no breasts at all, and i doubt you'd find much help with removing them as long as you identify as a woman. unfair, i think, but that's just how normative binary minds work. a man who identifies as a man won't get a neo vagina easily either, even if he genuinely wants one, and knows it's right for him.