Doubts are natural. I'm a year and change into HRT, out to everyone, starting RLE, like, today and I will still occasionally be like "Whoa, what if I'm not trans." All evidence suggests I'm as trans as a very trans person, but the conditioning to accept as etched in stone the sex I was assigned is incredibly strong. If you're told all your life that you are an X, and you are moving to Y, it is only natural to worry that you are X.
Typically cis people don't worry about this sort of thing, so consider that. But as others have said, transition is a buffet, take what you need and eat at your own pace. If you wanna try living as a boy for a while, do so, and if you like doing it, keep doing it. If you don't, then stop doing it. You may have some arbitrary deadline to present for the school administrator's sake, but that isn't the day you settle your gender quest for all time.
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