Source identified1
“For of all sad words of tongue or pen, The saddest are these: “It might have been!””
Whittier compresses regret into the imagined alternative life: both Maud and the Judge later measure their ordinary lives against a possibility neither chose when they briefly met.
Source note: “Maud Muller,” in The Works of Whittier, Volume I, Project Gutenberg #9567. The poem first shows both characters imagining the life they might have shared, then calls “It might have been” the saddest formulation before ending with a separate note of hope.