Regret Is Repentance

The Prophet said:
الندمُ توبةٌ
“Regret is Repentance.”

[“Ibn Majah”, 4252 – صحيح].
https://sunnah.com/urn/1343920

The dirtier you get the less bothered you are by dirt.
The cleaner you are the more bothered you are by dirtiness.
It’s the same with sins.
Regret is a sign that our heart still contains some good.
Not being bothered about sins at all, means that our heart is fully contaminated due to our sins.