Okay, panic not.. I got an E in my December mocks for history in year 11 but pulled off an A* for the real thing.
With maths, go through your year 10 syllabus and work out where you messed up. Use videos online/your tutor to ensure that you understand the concepts and then start doing practice questions. (do loads and make sure to ask someone is you don't understand it.) Don't start revising for the year 11 exams as they're new concepts so you'll need a teacher to explain them to you. What other subjects are you struggling with?