Final answer:
The sentence 'Regret over missed opportunities or actions not taken can be more sorrowful than other expressions of sadness' best captures the essence of Whittier's aphorism about the profound grief that comes from considering what might have been.
Step-by-step explanation:
The aphorism 'For all sad words of tongue and pen, the saddest are these, 'It might have been.' by Whittier suggests that regret over missed opportunities or actions not taken can be more sorrowful than other expressions of sadness. The melancholic tone of this aphorism indicates that looking back on what could have been done differently and realizing the potential for a different outcome can be a source of profound grief. This theme of regret for the unfulfilled possibilities is evident in various texts, where characters reflect on their past choices, wondering how their lives might have differed if they had made different decisions. For example, the sorrow of unfulfilled love, the pain of unseized moments, and the tragedy that could have been averted if only different paths were taken resonate with Whittier's sentiment on the gravity of 'what might have been.'