There is a story or a belief, I'm not exactly sure which, but maybe it's a principle, that highlights that our physical capacity to do more is limited by our perceptions of fatigue and not the reality of our muscular strength and endurance.  In fact, our ability to do more beyond our "fatigue point" is often 20% - 30% more.  This means, for example, that if you are doing push ups and you can only get 10 repetitions completed, that there is untapped strength, endurance and potential to do at least 2 -3 more.  There is another principle in physical training that highlights that pushing yourself to physical failure is where the majority of the physical growth and development occurs.  Getting better doesn't occur on the first or second repetition of the set where the movement feels easy and free; getting better happens at the last repetition where maximal exertion occurs and failure is imminent or actually does occur.  All the reps preceding this last maximum effort is simply preparing you for the failure.  Athletes of all kinds understand that the improvement zone occurs in the failure zone.  And this principle translates to other aspects of our lives as well.  Our greatest improvements occurs in our failures and the learning that we take from them.  

Try again. Fail again. Fail better. Samuel Beckett

All my successes have been built on my failures. Benjamin Disraeli

Those who dare to fail miserably can achieve greatly. John F. Kennedy

If we will be quiet and ready enough, we shall find compensation in every disappointment. Henry David Thoreau