I am a 11th standard student preparing for IIT JEE. I get jittery while facing Physics and Chemistry problems. How can I overcome this?

A lot of high school students face these issues, so it is nothing to worry about. Physics requires one to understand very fundamental concepts at a deeper level. It is important to be patient with oneself and focus on clearly understanding the concepts and practicing problems that test that understanding. This will improve both your understanding of the subjects as well as improve chances of success in the JEE.