What mental shift does henry wadsworth longfellow make in the poem "the rainy day"?

The poem The Rainy Day by Henry Wadsworth Longfellow tells the story of an older man who is having one of many depressing days. He is depressed because his youth is lost, yet he continues to dwell on the past instead of looking to the future. Eventually he realizes that everyone has sorrowful moments in their life, and must learn to deal with them and move on. At the end of the poem he develops a sense of hope, which is one of the themes of the poem. The speaker is speaking directly to the reader in 3rd person omniscient point of view. The tone of the poem is gloomy and dark, but at the end the speaker finds hope and the tone changes to hopeful because he realizes that this kind of thing must happen to everyone and he has to learn to deal with it.