Even though a mediocre test grade from decades ago has no bearing on my life now, I still dream occasionally about how it felt to receive a grade that wasn't as good as I'd expected. My mind softens the blow by reminding me of the only optimistic lyric in "Think For Yourself" by The Beatles. The general vibe of that George Harrison-penned tune is that it's too late to make amends. He offered one ray of hope, however, when he sang, "The future still looks good, and you've got time to rectify all the things that you should."
If you're a student, cling to that outlook--but study harder if you can.