Write down key phrases and memorize them with a positive picture. When you are in a situation where you need them, breathe deep, repeat the phrase and visualize the picture. Eventually you will just feel the picture and then you can work on a different area.
For example: When your stomach knots up and you start to get the doom feeling.
Say, "Tomorrow this will pass" or "Dread is fear internalized. I am stronger than my fear." Picture-water flowing calmly, swirling around the rocks, the sound, the breeze in the tree leaves. Repeat until it is a habit.