The concept of the Courage to Be Imperfect was developed by Dr. Rudolf Dreikurs. When we have the courage to be imperfect, we can focus on the present time rather than worry about what has happened in the past. Excerpt form a speech by Dreikurs at the University of Oregon in Eugene (1970):

To be human does not mean to be right, does not mean to be perfect. To be human means to be useful, to make contributions — not for oneself, but for others — to take what there is and to make the best of it. . . . We have to realize that we’re good enough as we are; we never will be better, regardless of how much more we may know, how much more skill we may acquire, how much status or money or what-have-you. If we can’t make peace with ourselves as we are, we never will be able to make peace with ourselves. This requires the courage to be imperfect; requires the realization that “I am no angel, that I am no superhuman, that I make mistakes, that I have faults. But I am pretty good because I don’t have to be better than the others” — which is a tremendous relief. . . . If we learn to function — to do our best regardless of what it is — out of the enjoyment of the functioning, we can grow just as well, even better than if we drove ourselves to be perfect (p. 289)

Source: AdlerPedia