You strengthen your lungs by pushing them: running faster and longer than you usually do to improve your conditioning. Your lungs’ capacity increases.
Strengthen your physical muscles by doing the same thing: lifting weights that are slightly heavier than what you’re used to lifting. Your muscles adjust over time and get bigger and stronger.
Emotional muscle? Same thing: push yourself to do things you don’t feel like doing and you develop the habit.
Very simple, right? Then what are you gonna do today?