If you believe that you’re bad at one particular subject and you revise by spending all day reading through your whole text book, two things will happen: you won’t get much better and you won’t feel any better about yourself.
Focusing your attention on only one problem area at once makes them seem far less intimidating. If you take a small sub-section from your syllabus, it’s possible, with good technique to go from low scores at the start of the day to measurably higher scores at the end of it. This builds your confidence and helps you to see progress as you work through each section.
If you take this approach ensure that you dedicate some time to review what you’ve learned over the following couple of days and then periodically after that.