In the song, the protagonist remembers bits and pieces of his past, feeling sad and nostalgic about his lost love and the hopelessness of his life. At the same time, he also expresses his confusion and uneasiness about the future, not knowing how to face the setbacks and difficulties in life.
Overall, "Sad Waltz" is a song about sadness, sorrow and confusion about the future. It conveys the song's protagonist's thoughts about life and expectations for the future through the dance form of waltz.