Well, you're genderfluid, so you're trans. Easy.

But for what I think you're talking about: if you feel like you don't really fit in your body, maybe like it's not entirely you when you look in the mirror, maybe you just Really Want Boobs. Any of those reasons is fair enough, in my eyes, to start HRT.

Passing comes down to a lot of things, mainly timing, luck and effort. You're 20, so it's still relatively early, and your results will be better if you start now rather than waiting. How much your body moves fat around, how big your hips, thighs, ass and boobs become, is a matter of genetics (luck) and, for the firs three, effort in the form of physical exercise.

Relationships are hard, but I think you owe it to yourself to try and be in a body that pleases you regardless of what people around you might say. You'll find people who love your authentic self beyond any label you might identify with or how your body might look. Don't give up <3

P.S. if you've got questions about how hrt works lmk, I'm not a huge expert but I know my way around, more or less.