I would like you to write down on paper, your areas of interest, your aims for the next exams and your study schedule. Once you have done that, please paste these papers on a wall, or keep them at a place where you can see them often.
And now, it's time to find out why you are not able to concentrate. As mentioned in one of the replies, there could be various reasons why your concentration is not up to the mark.
Find out what your ideas are about the exams you are preparing now. Did you always wanted to top all exams? Do your parents, other family members or friends desperately want you to top this exam, too? Are you looking for a guarantee of a good performance given your background of good academic career?
You could be carrying an underlying depression. It could even be an excessive anxiety about future (in general) and the exams (in particular).
Also find out if you have the problem of not being able to concentrate, in other areas of your life, too? Like playing, watching an interesting TV show, etc.
Get back here for any further queries.
All the best.
Dr Abhijeet Deshmukh, MD
Keshav Hospital, Nagpur