“I got a C” is a fact. “I’m stupid” is a story you stapled to it.

One of those two can be checked and the other cannot. A grade measures one piece of work on one day, under whatever conditions you happened to be in. The conclusion about who you are is an interpretation that arrived so fast it felt like part of the result, and it is the part that sticks around causing trouble.

Separate them on paper next time. Write what happened in one line with no adjectives, then write what you concluded, then ask what evidence actually supports the second line. There is usually much less than it felt like.