The essence of repentance is not a specific action.
It is not a recipe that one follows: so much charity, so many self afflictions, so many fasts.
Essentially, repentance is a feeling of the heart—regret over the past and a resolution for the future.
The greater the depths of a person’s mind and the development of his maturity, the more clearly he can recognize his problems and the more profoundly he can see each flaw.
And then his previous repentance may no longer seem to be enough, for he looks downward to levels of imperfection that his previous repentance had not been able to reach.
–Rabbi Adin Steinsaltz
From Understanding the Tanya, p. 51, by Rabbi Adin Steinsaltz