Exams can be a stressful time for any of us regardless of age, and just because you may not be at school any more, you're still going to come up against them.
Normally, your pre-exam nerves are worse than they are when doing the exam, but by the time you're taking the exam, you've worked yourself up into such a state that it seems just as stressful in the exam as-well.
Having too much stress can be harmful for your health, and will probably not allow you to perform at your very best. Having said that however, controlled stress can allow you to focus and reach your peak performance.
One of the best steps you can put in place right now to help with exam stress is to revise. Revising will help (and overcome if done correctly) any uncertainties you have and strengthen your knowledge.
If you're having trouble revising, then read a previous post of mine: 11 Tips to Revise Effectively.
Although it may not sound like it'll help, it will. Short and shallow breaths will cause your heart to beat faster than usual and will not help you control your stress. Instead take deep deliberate breaths to help keep your heart rate to a natural rhythm and to help with your anxiety.
Doing regular physical exercise is a recognised way of coping with stress. Just 10 or more minutes a day can make a big difference. It doesn't even have to be strenuous, it can just be something as simple as walking.
If you're tired and haven't been getting enough sleep, anxiety can be made worse and blown out of proportion. It's a good idea that you get enough sleep.