First, take a slow, deep breath.
Inhale through your nose and exhale through your mouth. Visualize yourself in the most peaceful place you can imagine - real or fantasy. Nothing exists but you, the space, and the sense of peace.
Visualize your stress and worries into a little ball you can crumple into your hands, then simply let it go. Choose not to let it take over you. You are bigger than this. Life is bigger than this. Everything will pass, and you can deal with things as they happen.