[...] I am not in the least troubled because you are discontented with your artistic productions. A certain amount of criticism must always play its part at each stage of our artistic development. Therefore I cannot see that the severest criticism can do any harm, nor that weak, superstitious and parasitical criticism can do any possible good to creative art. You will not indeed altogether forget criticism as you work, until you are impelled to production by the very life-force within you. Life, however, must be lived—and it still lies before you. So, do not get low-spirited. Contentment will come to you as soon as you cease to seek it in the sense in which you are bound to do now during your years of artistic apprenticeship and technical exercise!
Also do not envy Karl so much. If I am troubled for anyone I am troubled for Karl. He is incomparably farther than you from the living source of all content, from actual living. In his case I am continually asking myself, "When will he begin to become a living person?"—[...]
Source: Letters of Richard Wagner, Vol. 1, selected and ed. Wilhelm Altmann, trans. M. M. Bozman (J. M. Dent & Sons, 1927), No. 213