I used to get a little sad over grades, but I stopped caring about grades a while ago. Like Vomelche said, grades don't mean anything in the grand scheme of things. Grades are just meaningless numbers/letters that have no relevance to your life. Additionally, bad grades aren't even your fault sometimes; maybe you got a bad grade because the teacher was a dick, lost an important assignment, plays favorites with students, etc. Don't stress over grades. The next time that you get a bad grade, just think "whatever", and don't let it affect your life too much. Heck, just laugh at that bad grade, and say "lol, I got a 58 on that assignment".