If you want better results while studying, revision is the key. Repeated revision consolidates the information in the brain and helps to retain the information. Other than that, try a daily exercise of recalling your entire day in as much detail as possible before you sleep. Initially only important or superficial details are recalled, but with practice minor details can also be recalled. Be more organized; make lists. Use acronyms and mnemonic devices to help you remember. Take 45 minute slots followed by a 5-15 minute break. Eat and at proper times. Drink plenty of water. Find time for exercise or meditation. Keep time for proper revision at end of study session. Revise before you sleep. Wish you all the best for your exams.

This can be obsessive compulsive disorder where you have difficulty in finishing tasks at hand because you want everything to be perfect, you keep on obsessing about one topic and are not able to shift to another, try to just keep reading each topic twice without thinking about remembering it, try this if it doesn't work consult a psychiatrist who will give you medicines to concentrate better