thamizhvaanan,
"...what you are doing on forum"
It's an escape - something to take you away from what you dread at times due to nervousness etc. When you feel like that do something different from what you should be doing and what you turn to do instead. That will take you back to what you should be doing. Do this a few times and you would get rid of such a problem. Talking from experience :D This is easier said than done, but try it and not only with exams.