I had a setback this week with my running due to a number of things culminating in pain in my shin and calf. So I had two consecutive days that I did a total of four miles. When that happens less than three weeks out from a marathon, the tendency is to panic. Fortunately, I have had so many setbacks over the years that I know setbacks are just part of the game when you approach your current limits.
How I react has changed over the years. My first injury in college reduced me to tears one mile into a run. I cried as I walked a mile back home with excruciating knee pain. Now I greet setbacks with an appropriate overreaction. Typically that overreaction involves drinking more milk, stretching, doing the little strength exercises, eating a variety of vegetables, and sleeping more. Basically three days of that and I'm good to go again.
On the mental side of setbacks there is a lot of doubt that one can have about one's abilities. For example, I have not done a workout in a week and a half, I'm scaring myself that I am out of shape. However, the facts are I have had my best three long runs, two best 20 minute tempos, one and only special block, best pace variation tempo, best mile repeats, and two days with over 30 miles. I'm in the best shape I have ever been in.
Similarly I had doubts at work last week. My grandma died and one day that week I said probably two sentences all day at work. I felt like I was in a dead end anti-social situation wasting my life behind a computer screen. Then this week we had a number of social informal discussions at work and it revived my attitude. I have trouble being patient sometimes, but it is typically to my advantage to be patient.
All of these life experiences help me to doubt less. What will happen will happen and worrying about it will not make it any better. Feelings are not fact. When I feel insecure about something the facts are that I am likely more secure in that situation than I feel. At the same time, doubts help us question ourselves and refine our thoughts. I suppose that doubts are the process of refining our thoughts until there are no longer any doubts.