Hi sandeep yes I understand its very important to choose a right pen for writing exam. The most important thing about the pen is that it should be slim and lightweight. My suggestion is to use soft tipped pen and not the 0.5mm sharp tipped ones by which you are getting irritated, because these may tear the paper while u are writing fast and also after sometime the words tend to be lighter. While writing don't put cap on top. The pens Cello Technotip, Cello Pinpoint-XS, Cello Finegrip, Cello Gripper , Cello Maxriter F, Reynolds 045 Fine Carbure and Nataraj Classic Fine Ball Penare relatively lightweight except Cello Pinpoint and Cello Gripper and these two pens start hurting the joints of the thumb.
You try using Reynolds force gel because it is neither too thick nor too thin,its waterproof and gives appropriate friction.
Atlast I suggest you to try some 6-10 pens and use each one for atleast 3hrs continuously before coming to a conclusion.All the best for ur exams.