In what way are you smart? I started to think about the things I was good and bad at, and I realized I'm only good at certain types of problems and work due to practice and study. Try to be confident in your interests and pursuits where you know you have experience or you want to gain experience, but don't be overconfident and begin think you know about things that you barely scratched the surface of. A good start is to never assume you know how to solve a problem that has been unsolved for ages, in general. Often, the people who hang out here don't know that line because they think they're "smart".