Acidity is such a sharp way to put it, and it's the whole engine for me. Without that bitter, sour sting of regret, a second chance feels weightless, like they're just picking up where they left off because of convenience. The acidity is what makes the characters flinch when they remember—that specific, gut-punch memory of the exact moment they chose wrong. It's not just sadness; it's self-loathing. It corrodes their ego.
In 'The Unwanted Wife', the hero's regret isn't gentle. It's acidic. He spends chapters practically burning from the inside out with the knowledge of how cruelly he treated her, and that corrosive feeling is what fuels his desperate, often clumsy attempts to make amends. It makes the grovel feel earned, because you can taste his misery. The sweetness of the reunion only works because the preceding bitterness was so potent.
If the regret is just a mild 'oops,' I lose interest. Give me that visceral, teeth-gritting, 'I ruined the best thing I ever had' acid bath. It's what makes the healing process feel like a real struggle, not a foregone conclusion.