As of today, I have been a student for the past 14, all the way from Pre-K to sophomore year in college. It has been a very long time with lots of changes, but one thing that has stayed the same has been that I have almost always been graded. It started off with the easy stuff like reading and then got a bit more advanced with the division, and now I am onto the hard stuff like calculus and microbiology. Regardless of the content, a letter was always attached to how well I was doing, A, B, C, etc. But, over time that letter did not only measure my "understanding of the course material" but also my self-worth.
Time and time again I have heard students lament bad grades with statements like "omg my mom is going to kill me" or "wow no one is going to want to hire me if I fail this course." A single bad grade suddenly becomes something that is so much bigger than a class, let alone a single letter of the alphabet. More so, nowadays the expectation and range of which grade to get have gotten narrower and narrower. Now, getting a B is simply average, while getting an A is what is thought to be as the best.
Obviously, within the moment, a bad grade feels like the end of the world. It feels like all of your hard work and effort is not good enough, and will never be good enough. Now clearly this logic is melodramatic but the constant level of stress students face can easily explain why a sudden failure would be so much more painful. If you are on edge 24/7, there comes a moment in which you have to breakdown.
But, as people there is so much to us than what a letter can describe. You can be loud or quiet, artistic or scientific, or maybe a mix of them all. Maybe you are horrible at writing essays but amazing at writing poems. Maybe you love chemistry but hate biochemistry. Maybe traditional education isn't your thing, but instead, you are great at a specific job or hobby.
At the end of the day, a bad grade is something that most people just forget about over time. That specific project or even class blurs away and becomes inconsequential in the long scheme of life. Your mom will still love you, you will still be able to get a job, and you will still be able to be proud of yourself. Think about it like this, are you really going to let such an inconsequential thing like a letter be how you measure your self-worth?