Not Me Thinking I've Thought Of Some Original Awesome New Concept

That crushing moment when your "revolutionary" mathematical insight was actually discovered by some ancient Greek dude wearing a toga. Nothing humbles you faster than learning your brilliant epiphany about prime numbers was thoroughly explored by Euclid in 300 BCE. The mathematical universe is just one giant game of "too late to the party" where Newton and Leibniz are still arguing about who invented calculus first while you're in the corner thinking you've discovered something by doodling during a boring lecture. Even Einstein had to deal with Lorentz being like "yeah, I kinda already worked on that transformation thing." The history of mathematics is basically just a timeline of brilliant people saying "I thought of it first!" followed by librarians saying "actually..."