Hello Mohith

Generally this happens with students during exams. Even it was the same in my case. The reason why you are not able to perform in your exam when you  already have practiced the same before- can be either you have studied too much or you are very stressed because of you get confused and fail to perform. If you have practiced MCQs, your concepts are clear + basics are strong then why to worry.!?  Don't stress yourself and stay calm. Say to yourself that you know everything and you can perform greattt. Keep confidence and don't overstudy. Read, revise, practice and go through the formulae chart. Simple..!
Set your target and  aim on scoring maximum possible.
Give your Bestttt.!

Good Luck.!