I have dreams of going to the most competitive schools in the country, but I think I sabotaged myself by getting a C in AP Calculus. I don't want to be a science/math major, so that's probably good, but I still feel like the C might be a dealbreaker. I have all As otherwise. How worried should I be?
If you're qualified in terms of test scores and extracurricular accomplishments, don't worry too much about the C. Since you have all As otherwise, your overall GPA should still be high enough to qualify you for admission at very competitive schools. One grade won't make or break your admissions chances, especially if the school thinks you show lots of promise and passion in your other pursuits. If you're still worried, you can use the additional information section on the common app to explain any extenuating circumstances that may have led to the lower grade.
Here's a quote from the Princeton admissions website to highlight my point:
"Instead of worrying about meeting a specific set of criteria, try to create an application that will help us see your achievements — inside the classroom and out — in their true context, so we can understand your potential to take advantage of the resources at Princeton and the kind of contribution you would make to the Princeton community."
No college will discard your application because of one low grade.