Regret

[Sarah]

Regret is wishing you had said something when you had the chance.

[Hina Muqadas]

A group or gathering of ghosts; attributed to Christopher Moore, author. First Known Use: 2015, First sentence, Chapter 6, Second Hand Souls

[Greta]

Feel sad, repentant, or disappointed over (something that has happened or been done, especially a loss or missed opportunity)

[BertababySog]

sadness associated with some wrong done or some disappointment

[EdwardyHuche]

feel remorse for; feel sorry for; be contrite about

[SanchoPn 0]

decline formally or politely

[Emorynaseaps]

To feel sorry about (any thing).

[ArthuraCig]

Prior has used it in the plural; but, I believe, without authority.

[sastergri]

pain of mind on account of something done or experienced in the past, with a wish that it had been different; a looking back with dissatisfaction or with longing; grief; sorrow; especially, a mourning on account of the loss of some joy, advantage, or satisfaction

[soniportgo]

express with regret