Fetching details from the dictionary...
regret; remorse; repentance; contrition
悔恨の念に苛まれて眠れない夜が続いた。
Tormented by feelings of regret, sleepless nights continued.
あの時の判断に対する悔恨は今も消えない。
The regret over the decision made at that time still has not faded.
悔恨の涙を流しても過去は変えられない。
Even shedding tears of remorse cannot change the past.