It is true that if you want to be a girl, you can be one; and if you don't want to be one, you can decide not to be one. And the same is true of boys. But I want to posit something else: that you don't need to buy a whole package deal. You can pick and choose the traits and words you like.
I conceptualise gender and masculinity/femininity not as boxes that one needs to conform to, or labels that one has to choose to adopt or reject, but as toyboxes full of trinkets. I can take whatever trinkets I like. I don't have to take the ones I hate. If I hate all of the toys in a box, I can just close the box. If I like only some of them but hate others, I don't have to bring the whole box with me. I can put them back in the box if I don't want them one day, and I can take them back out if I want them again later. I can play with the toys and paint the toys and mash the toys together and experiment with the toys and break the toys and cherish the toys at my leisure. And I can buy them from elsewhere, or even make my own, if I want more toys but can't find them in the toyboxes provided.
You are under no obligation to follow the rules of gender. You are under no obligation to pick an entire set of traits and stick with it forever. You are under no obligation to avoid things that sound contradictory to others who don't understand what you're doing. You can be a girl who uses he/him pronouns and takes T. You can be a boy all the time except on Tuesday at 5pm when you become a girl for a little while. You can wear a dress with your waistcoat and a big pink bow with your big curly mustache and give yourself it/its pronouns and be a lesbian and a gay man at the same time. You can decide you hate all of this shit and throw it all out the window, and you can be any of these things sometimes and something else later. The list goes on. There is absolutely nothing stopping you. There are no rules. You can be a gender scavenger. Try it out and see how you feel.