It's often possible to radically change your stress levels whilst in the air. If you are with an instructor don't beat yourself up every time they say you are doing something wrong. Instructors continually "raise the bar" - so they are expecting a higher standard from you because you are getting better.
After you have your PPL and you are flying solo you may need to reduce your workload if you are feeling stressed. Adjust the radio so that it's clearer, less noisy or whatever, make sure you are sitting comfortably. Don't feel you have to announce yourself to every en route ATC if flying VFR - or find an ATC which is going to be with you for most of the flight so that you don't have to continually change frequency. Don't fiddle unneccesarily with GPS units. Slow your cruise speed if the a/c is getting ahead of you. Concentrate on the basics - i.e. flying the aircraft.