regret
n. C / Un. a sad or sorry feeling about something you did or did not do in the past. You feel this when you wish you could change a previous choice.
n. a feeling of sadness, repentance, or disappointment over an occurrence or a missed opportunity. Often used in the plural to express formal apologies or polite refusals.
He feels a lot of regret about leaving his old job.
She looked back on her university years with a sense of regret for not studying abroad when she had the chance.
The aging statesman expressed profound regret that his diplomatic efforts had failed to prevent the outbreak of hostilities, though he maintained that his intentions were always peaceful.
From Middle English regretten, regreten, from Old French regreter, regrater (“to lament”), from re- (intensive prefix) + greter, grater (“to weep”), from Frankish grātan (“to weep, mourn, lament”), from Proto-Germanic grētaną (“to weep”), from Proto-Indo-European ǵʰreh₁d- (“to sound”); and Frankish greutan (“to cry, weep”), from Proto-Germanic greutaną (“to weep, cry”), from Proto-Indo-European gʰrewd- (“to weep, be sad”), equivalent to re- + greet. Cognate with Old High German grāzan (“to cry”), Old English grǣtan (“to weep, greet”), Old English grēotan (“to weep, lament”), Old Norse gráta (“to weep, groan”), Gothic 𐌲𐍂𐌴𐍄𐌰𐌽 (grētan, “to weep”). More at greet.
Often followed by the preposition 'for' or 'about'. When used in the plural ('regrets'), it frequently functions as a formal way to decline an invitation.
I have a big regret for my mistake.I have a deep regret for my mistake.While 'big' is common for physical size, abstract feelings like regret are typically described as 'deep', 'profound', or 'great'.