There is a statement in מסכת ברכות that is very fascinating. Rabbi Yochanan says in the name of Rav Yossi: טובה מרדות אחת בליבו של אדם יותר מכמה מלקיות, that one’s personal regret in his heart, is worth more than many sets of lashes.
The Gemara is telling us that the most effective Teshuva that one can have, is when one recognizes on his own, the need to fix his deeds. It is nice when one is able to be moved by the rebuke of a holy Rabbi.
However, if one suddenly realizes on his own that he needs to get his act together, and he cannot go on living as before, it is as if he’s been to court and he received lashes. Reish Lakish adds that such self introspection is as if he received 100 lashes.
The Pasuk proving this, comes from Mishlei where it says, “Chastisement frightens an understanding one, more than smiting a fool one hundred times.” The Pasuk is referring to self-chastisement which is the most effective way of Teshuva.