You'd never allow a thief in your house. Why permit thoughts which would rob you of your form?
Worries, doubts and hesitations thieve you of your most valuable asset: focus. You can't worry your way to a solution, doubt your way to certainty, or hesitate your way to action.
No matter the stress in your life, increase the difficulty of an exercise to improve your mental self-defense against excess stress. The higher the degree of required focus, the greater you exercise your strength of will.
Exercise isn't about moving your body, but about focusing your mind. Your physical fitness will come and go, but only your mental focus reclaims it; and grows stronger each time that you do. You don't find the will. You create it.