There is a difference between working hard and working smart- I learnt this the hard way at your stage. You can work for hours reading textbooks etc, but still not do well overall. The process really is a matter of understanding how your brain works and how it retains information.
for starters, do you revise through hours of continuous study, or through short breaks and recaps during the process. The latter actually has more value because it assists in the amount of information one retains in the brain over the long run, rather than a long-term period of consistent cramming of information.
secondly, how well do you do on past-papers. usually, they are patterned so you know what youll probably expect. If you find it difficult however, to apply theories from the textbook, do you understand the theory? Do you actually know what the purpose is, why it is the dominant theory etc?