It’s helpful to read around your courses.
My college had a policy where we were expected to do 5 hours per subject extra work per week (so 15 hours a week of extra reading and work) although I’m not sure whether your college/6th form will have the same policy: but it works for getting the best grades
What topics are you doing in history? I did Tsarist and Communist Russia, Britain 1951-2007, and the French Revolution - history will be quite difficult I can guarantee but so long as you put extra effort into your subjects it’ll pay off, trust me!