Courtesy of Letters of Note, the first from a series of letters from Rainer Maria Rilke that would later be collected into Letters to a Young Poet:
You ask whether your verses are good. You ask me. You have asked others before. You send them to magazines. You compare them with other poems, and you are disturbed when certain editors reject your efforts. Now (since you have allowed me to advise you) I beg you to give up all that. You are looking outward, and that above all you should not do now. Nobody can counsel and help you, nobody. Search for the reason that bids you write; find out whether it is spreading out its roots in the deepest places of your heart, acknowledge to yourself whether you would have to die if it were denied you to write. This above all—ask yourself in the stillest hour of your night: must I write? Delve into yourself for a deep answer. And if this should be affirmative, if you may meet this earnest question with a strong and simple, "I must," then build your life according to this necessity; your life even into its most indifferent and slightest hour must be a sign of this urge and a testimony to it.
Link to the rest here.