I listen to the lectures very carefully but when the time comes for representation im nt able to why?

actually when i come from college i will plan to read this subject and that chapter but when i open the book i feel bored or some thoughts will be disturbing ,or even sometimes i feel proud "aah this answer only naah,or sum i heard the lecture i will read this before the exam but no use pls help me how to control this stupid activity of mine and get full concentration