Working harder will not get you further if you keep repeating the same mistakes. Do you have any examples of where you are losing marks?
Getting a C grade at A-level, does mot mean you are failing, It means you achieved a pass but have not yet mastered the art. The issue is raising your grade. To do that, you need to get a better insight and understanding of the concepts.
I'd suggest You must already know that Physics is all about conceptualising and describing the 'behaviour' of observed phenomenon. The way that behaviour is described, is in the language of mathematics. But that can be as much of a hindrance as a help if the student cannot describe to others in words what concept the maths embodies.
It will be very worthwhile reading beyond the syllabus, not so much trying to answer harder questions (although that will help a great deal), but start by gaining an understanding of what the mathematics is really describing.
For instance, the equation F = ma, is easy to remember, but what does it really mean? Many people simply regurgitate the equation with no further thought. Fine for a C grade, but inadequate if you want that A*.
Where did the equation come from? i.e. Is this the original form or is it a précis of something else? How does this equation relate to momentum, specifically the acceleration part describing change in momentum?
Try answering other peoples questions on TSR, not the part that simply gets a numerical answer, but what the equations mean and what they are trying to describe.
You need to get to a point where you can deconstruct any problem without having to memorise solution steps, but by understanding what concept the question is aimed at you demonstrating your understanding and working through from first principles.
Post you questions in the Study Help forum and I'm sure you will get the help you need to better your grades.
You can do it.