The following is a freely translated except from a letter by the Rebbe, dated Adar II 17, 5717 (AdarII 17, 1957) 1.
… as to what you write regarding [your] depression—the Holy Tanya 2 has already explained that all depression stems from an unholy source 3. In any case, how is it that such a matter concerns yourself? I wonder: How does a chassid have time to be depressed? Does one not realize that time is now the most precious of resources, its every moment an asset that must be utilized to the utmost? How much time is being wasted on matters for which “petty” is too generous an adjective!