Well, ask yourself, "Do I try hard enough?" "Was it the best I could do?" And well if the answer is yes, then you have nothing to stress out about too much. Maybe talk to your teacher and ask for extra credit. Get tutoring. Talk to a guidance counselor. If none of that helps, well then you're just not the best at the subject, but just try your best. In the future, you won't even remember about all those grades! Coming from a 6th grader, the beginning of the year was super hard. I'm not the best at math, but I'm very good at Language Arts. Everyone has their own weaknesses and strengths. Don't worry about it.
