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Variant: I have made it a rule of my life never to regret and never to look back. Regret is an appalling waste of energy, and no one who intends to become a writer can afford to indulge in it. 
Context: I have made it a rule of my life never to regret and never to look back. Regret is an appalling waste of energy, and no one who intends to become a writer can afford to indulge in it. You can't get it into shape; you can't build on it; it's only good for wallowing in.
                                    
 
        
     
                             
                             
                             
                             
                             
                             
                            