To the Lethean peace of the skies— (line 46)
Lethe is a Greek word meaning "forgetfulness" or "oblivion." In Greek mythology, when people died, they would drink from the river Lethe, which would make them forget about their past lives. We think it's pretty significant that, when the speaker dreams about heaven, he doesn't imagine music or angels or happiness. He just thinks of it as a place of forgetting. This seems like another clue that he's in pretty bad mental shape. If you were feeling healthy and balanced, you probably wouldn't fantasize about forgetting everything you ever knew.