When you’re stressed, you tend to breathe less deeply. As a result, your brain gets less oxygen, which it interprets as a sign of stress. That causes you to breathe even more superficially. A way to break this vicious circle is to breathe from your stomach.
How do you do this? Put your hand on your stomach. Try to breathe deeply starting from there. When you breathe in, your stomach should get bigger. When you breathe out, your stomach should go down. Watch this video for more information about breathing from your stomach.