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.
These matters are not known to everyone but only to "the knowing."
It is only the self that can recognize the state of one's heart–and even then only when judging honestly, at every moment and in every circumstance.
When a person views his soul in this way, so that he knows it and senses it, the heartfelt sentiments of regret and repentance that had been enough at an earlier age and at a different level of being will now appear superficial and unsatisfactory.
The greater the depth 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 imperfections that his previous repentance had not been able to reach.
–Rabbi Adin Steinsaltz
From Understanding the Tanya, p. 52, by Rabbi Adin Steinsaltz