When I think I'm just too tired to exercise, I commit to do 10 minutes of light exercise. If I'm tired after 10 minutes, I'm allowed to quit. But 95%+ percent of the time I feel better after 10 minutes and go on to get a good workout.
I think of my plan for healthy living in terms of a football game. A fumble isn't part of the game plan, but it is part of the game. If I slip, I just shake it off!! It has meant the difference between multiple attempts and success.