I was reading through this and perhaps I can offer advice which used to help in my studies.
I found the best way to revise for humanity subjects (e.g history, etc) was to read the topic several times over and then to cover the material up. Then I would try to re-write the essay from memory. After I had written what I had remembered, I would compare what I written to what was in the text book. The points I missed out I would write 10 overs. Then I'd cover up the material again and start to write what I could remember and then continue the process over and over again until I could remember most of the points. Then I would repeat the same topic next day and then every 2 or 3 days or even a week later.
For sciences and mathematics the best way is to remember the formula rather than cumbersome definitions. The best way here is to do many examples as possible. Past examination papers or model questions can help in this.
Also I think if you can an interest in the subject than you are far likely to succeed. I hope this helps.