My top lesson was realizing that I am a stubborn person, and that I was wrong to keep trying to implement unimplemented features past a deadline and that I need to understand when to give up. I also learned that I can't trust others to finish their part of something I start. There is nothing like seeing the entire backend you wrote be gutted by someone else because they "needed to learn how route handlers work by creating it themselves" and then seeing them not complete said route handlers.