In the long run grades don't really mean that much. My parents always expected me to get A's and even when I would get an A- they would ask me where I could improve. Don't strive for a letter grade, strive to learn, try enjoying it and maybe your grades will improve. Sometimes parents have such high expectations because they love us and think that we can do better, when getting a C might be an amazing grade for someone. Don't be too hard on yourself, whatever you do in life the Lord will help you through it.
I don't judge people on their grades, if you think about it it's just a system of a person judging the work of another. Try your best and the grade you get may not be the best compared to the worlds standards, but it's your best. Be proud of it.
When it came to grades and school, this was my goal:
Colossians 3:23
Whatever you do, work at it with all your heart, as working for the Lord, not for men