Regret
Part of speech: verb
Definition: To feel sorry about (a thing that has happened): to wish that a thing had not happened, that something else had happened instead. To feel sorry about (any thing).
Part of speech: noun
Definition: The instance of such an emotion.
Example sentence: We must all suffer one of two things: the pain of discipline or the pain of regret or disappointment.
Ruefulness
Part of speech: noun
Definition: The property of being rueful.