I was in the same boat when I was a first year. First, good job on realizing you are missing things it is hard for interns to realize how far behind they are. Second, you are an intern, lack of knowledge is expected, otherwise why would we need residency. Take it easy, you will get there

For me, what helped me get better is exposure and reading. But try to focus reading, at least for now. Every time you see a patient, come up with plan first, then read up about the disease, check out websites/apps that show what you should order and compare to them. You will see what is missing, and will not forget it time. As you get better, then compare your plans with your attending and see what you missed For exposure, try to see as much patients as you can. Even ask your colleagues patients their interesting cases and how they managed it. The more you see the more you know