"Where have all the flowers gone?" by Pete Seeger
The story behind the song speaks of loss, over time:
I had been reading a long novel—”And Quiet Flows the Don”—about the Don River in Russia and the Cossacks who lived along it in the 19th century. It describes the Cossack soldiers galloping off to join the Czar’s army, singing as they go. Three lines from a song are quoted in the book: ‘Where are the flowers? The girls plucked them / Where are the girls? They’re all married / Where are the men? They’re all in the army.’ I never got around to looking up the song, but I wrote down those three lines.
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The counselor added two actual verses: ‘Where have all the soldiers gone? / Gone to graveyards every one / Where have all the graveyards gone? / Covered with flowers every one.’
The song's title is evocative of the feeling you describe, of time slipping away.