Accidentally filed my taxes using TurboTax as a US citizen while I'm not

TurboTax is not "only for US citizens". TurboTax is for US citizens and resident aliens. Only if you are a nonresident alien should you not use TurboTax, since they do not support Form 1040NR.

You are a resident alien for US tax purposes if you pass the Green Card Test or the Substantial Presence Test. You pass the Green Card Test if you were a US permanent resident (green card holder) during the year. You pass the Substantial Presence Test if (the days you were present in the US that year + 1/3 the days you were present in the US the previous year + 1/6 the days you were present in the US the year before that) >= 183 days.

You said you were an F-1 student, and F-1 students are considered "exempt individuals" (i.e. their days on F-1 status are not counted in the Substantial Presence Test) unless they have been an exempt individual for some part of 5 previous calendar years. So if you just came on F-1 status in 2019, and you have not been in the US in F or J status before, you would be an exempt individual for all your time on F-1 status in 2019, and thus you would not pass the Substantial Presence Test for 2019 (unless you were in the US in another status for the part of the year before you came on F-1), and you would thus be a nonresident alien for 2019.

As a nonresident alien, you were not supposed to file tax returns using Form 1040, so you shouldn't have used TurboTax. As a nonresident alien, you are also not eligible for the stimulus check. You need to amend your tax returns by filing Form 1040X. The 1040X instructions has a section on changing from one form to the other. Basically, you only fill out certain parts of 1040X, and attach a correctly filled-out 1040NR. This IRS page has information on how to return your stimulus payment.