I did all the past papers but I still only achieved a C grade, what am I doing wrong?

Doing the Past Papers only gives you a general idea of what types of questions will come up in your exam. Every exam will be different and will have those particular questions which require knowledge of multiple chapters in a module and fundamental maths knowledge to solve, which is where grade C students are separated from A* students. The key is to find questions that you do not immediately know the answer to and spend the time trying to work it out and if you can't, then take a look at the answers but make sure you understand every element of the answer as each element will require knowledge from different areas in maths.