At mock season last year January I had gotten an E in my Maths paper.
In the real paper, I got a B, 3 marks of an A, and my mathematics is TERRIBLE.
The key was what 'my_name_' had said. Understand the theory. As soon as I got that B, I went back to square one, I'm talking about D grade mathematics. Build up from there. Understand how to round, becuase this can lose you marks otherwise. Understand basic D grade fractions, then move to C grade, then B grade. The reason this works is because it builds up your ability and if you have no basic maths foundation, there is no way you could answer the B grade questions because they encompass D-C grade thinking. If you learn them, you somehow manage to apply it to the hardrr questions and start picking out key concepts in harder questions that link to D or C grade topics. It took a Very long time and people laughed when I told them I had started D grade topics again but I swear to you it works. Luckily my school had invested in a website called MathsWatch (pretty sure you can buy it too). You can print out worksheets for every single maths topic from it (grade D-A*) and constantly work on them. Otherwise, HegartyMaths on Youtube was a LIFE SAVER.
I really hope you do well, because getitng a B or higher in Maths GCSE can open up a lot of doors in terms of university for you. Good luck, and start working from now!