When you figure it out, it really is like a lightning strike moment. All of those things youv'e done in the past, all those feelings of disconnect/discomfort that you thought were normal (what teenager DOESNT hate their changing body?), they're now Signs that you've been trans all along. It can be pretty overwhelming!
For some people, they want to "address" this and start making changes right away. Others want to wait and make sure they're 100% sure. Hormones and surgeries cause irreversible changes, but things like how you dress, how you do your hair, and what names/pronouns people use for you on the Internet or (very chill) friend groups aren't. Surgeries are the only thing where you have to PROVE you are trans and have "been trans" for x months/years, but that's because insurance is ass. Some states might require you to be trans or have certain procedures done before you can legally change things, but others there's no such requirement.
I don't think you're overreacting! You can start making changes today if you want. When you're getting into the medical/legal leg of it, though, make sure you do your research so you know what you're getting into, the costs involve, the various pains in the asses that crop up (dealing with used sharps, your credit score taking a hit when you get a new name).
Feel free to DM me if you have questions about masculinizing HRT or surgeries. I can give tips on insurance/the medical system, too, if you're in the US.
Also, do be aware that there are people that are stupid and say shit like "there's no point in transitioning after 30/28/26/25/21/18/15/16/13. It's too late!!!" Yes, if you want MAXIMUM results, you need to get on this before "normal" puberty hits, but the vast majority of trans people aren't able to do that (either they haven't figured it out yet, their parents/society weren't supportive, or they couldn't afford it). There's no such thing as it being too late to transition. HRT is nuts, it changes so many things. But, more importantly, it's replacing a vital chemical your brain needs that it isn't currently getting. And that'll make your life better in so many ways that psych meds or therapy can never do.