What does the midnight song "The past is empty, like a dream" mean?

This is Li Yu's later lyrics, which is one of the lyrics to relieve the depression after being captured.

Generally speaking, dreams are often substitute compensation for reality. In reality, we can't go back to our old country, so we have to return in our dreams. However, dreams are short-lived and illusory, and when we wake up, the pain of loss deepens. The dream was not long, but we turned to reminisce about the happy past, empty, and then deeply disappointed.