The GCSE year is a very difficult time for most students. You are probably studying a wide variety of subjects, all of which are competing for your attention when it's time to start revising. Maths is a somewhat unique subject, in that the skills required to pass an exam and achieve a good grade can ONLY be acquired through practice. This is simply not the case for many other subjects.
When I am preparing my students for their Maths GCSE, I truly cannot overstress the importance of practising as many different (and difficult) questions as possible, well in advance of the exam. Many GCSE Maths papers follow a similar format from year-to-year and it is the students who have familiarised themselves with this format who achieve the top grades.
Every student will, at some point, encounter a topic they find difficult. As a tutor, nothing pleases me more to see one of my students break through a particuar area of difficulty and master one of these topics. It is only by doing as many exam-style questions as possible, as early as possible, that a student can identify their strengths and weaknesses and be truly prepared for the exam.