Stress comes from the unknown. I wish I could give you specific answers WRT to the UNSW course. The medical *shouldn't* be stressing you at your age. There should be a document or two on the CASA's site about what's in the medical. Spend some time sifting through that to allay your fears. Essentially, eyesight and hearing should be ok, don't have diabetes or migraines etc.
What do you mean by overseas? Is this for holidays or for work?
Right now, every thing seems to be a massive rush and panic. You'll soon find that you've got a lot more time than you think.
Managing stress will be a key skill you'll need to manage to handle exams and other life pressures. The best way to manage stress is to take the time to prepare and study. Tick off the elements you need to achieve and that will help manage the stress.