remorse
A deep, painful feeling of regret for hurting someone.
Remorse is the deep, painful regret you feel when you realize you've done something wrong and hurt someone. It comes from genuinely wishing you could undo what you did because you now understand the harm it caused, not from fear of punishment or consequences.
When you feel remorse, your conscience is working. You might feel it gnawing at you for days after you lied to a friend, even if they never found out. You might lose sleep over it, replaying the moment in your mind and wishing desperately that you'd made a different choice. True remorse makes you want to apologize and make things right, not because you fear punishment, but because you genuinely care about the person you hurt.
Remorse is different from embarrassment (feeling bad because people saw you mess up) or fear (worrying about getting in trouble). Someone who accidentally breaks a neighbor's window might feel bad, but someone who breaks it on purpose and then sees the neighbor's disappointment might feel remorse, that sick, heavy feeling of wishing they could turn back time.