Changing research interest in Math
You haven't mentioned any alternatives to Number Theory.  Hmm.  Maybe it would be reassuring for you to have a small list in your hip pocket of possible areas.

If you don't have to decide right away, then I would suggest that you hold off on deciding, and meantime start remedying these gaps as soon as possible.  Without panic, but without any shilly-shallying.  Obviously, don't work on them all at once, but do start somewhere, and make a tentative map through this subject matter -- in what order, what books will you use, what topics could be skipped or skimmed over, which ones will you do in a class, which as self-study, and which in an independent study.

Even if you end up working on something other than Number Theory, this study will be helpful for you in the long run, either directly or indirectly.

If it feels daunting, think one step at a time, or Rome wasn't built in a day, or some similar phrase that helps you.

In the U.S., one is generally not expected to hit the ground already doing research, right from the first semester.  It's okay to take some time and get the solid foundation one needs.

Do you have an advisor, potential advisor, temporary advisor, or mentor of some sort?  If not, look around for one, and if necessary ask the department to assign you one temporarily.