Into the Silence. 
Broken Vessels (1991)
                                    
“If you think it cannot be done, please do not interfere or interrupt the rest of us trying to do it.”
1972
Related quotes
“Never interrupt someone doing something you said couldn't be done.”
                                        
                                        As quoted in "She Drew Horses..." (2006) by Kelli Swan, p. 42 
Disputed
                                    
“Don't let what you cannot do interfere with what you can do.”
Variant: Do not let what you cannot do interfere with what you can do.
                                        
                                        We can quite well turn away from our true destiny, but only to fall a prisoner in the deeper dungeons of our destiny. … Theoretic truths not only are disputable, but their whole meaning and force lie in their being disputed, they spring from discussion. They live as long as they are discussed, and they are made exclusively for discussion. But destiny — what from a vital point of view one has to be or has not to be — is not discussed, it is either accepted or rejected. If we accept it, we are genuine; if not, we are the negation, the falsification of ourselves. Destiny does not consist in what we feel we should like to do; rather is it recognised in its clear features in the consciousness that we must do what we do not feel like doing. 
Source: The Revolt of the Masses (1929), Chapter XI: The Self-Satisfied Age
                                    
                                        
                                        A Little Book in C Major, New York, NY, John Lane Company (1916) p. 53 
1910s
                                    
“Do as you would be done by, is the surest method of pleasing.”
                                        
                                        9 October 1747 
Letters to His Son on the Art of Becoming a Man of the World and a Gentleman (1774)
                                    
“Those who say it can't be done are usually interrupted by others doing it.”
“Rest satisfied with doing well, and leave others to talk of you as they please.”
                                        
                                        As quoted in The World's Laconics: Or, The Best Thoughts of the Best Authors (1853) by Everard Berkeley 
Variant: Rest satisfied with doing well, and leave others to talk of you as they will.
                                    
                                        
                                        "This Is Not a Test". 
She & Him : Volume One (2008)