You will continue to feel this way for a long time. You should continue to feel this way. They didn't make a mistake in admitting you, but your future professors and advisors know that you have a vast amount still to learn. I think it's healthy for you to know that too. Honestly, right now, you're probably not very good! Even if you've done a MA, if you've spent years in curatorial fellowships, whatever, being a good Ph.D. student is something that you need to learn by immersion. If you're going to a good program, that's what your next several years of coursework is for. You'll get there. Take risks with your term papers. Try on new methodologies for size as you encounter them. Maybe most importantly, don't worry about "keeping up" with your future peers, or even worse, competing with them. Most likely, some of your classmates will blow you away with how adept they are at certain things. You'll undoubtedly outpace them in certain aspects as well. Treat your colleagues as the most valuable resources that the grad school setting has to offer. When one of your classmates' seminar paper presentations makes yours look like a 3rd grade modeling clay diorama in comparison--this will happen, so many times--take them out for a beer and pick their brain. Instead of comparing your own work to that of your peers, consider work you admire as a model to examine. Why was it so good? What questions were they asking? How was it organized? How, when, and to what effect did they incorporate visual analysis? It took me longer than it should have to really understand this, but it's so, so good for you to have kickass colleagues.