He forgives the sins of His people, and passes them over, one by one ... (Selichos).
The Talmud states that if a person repeats a particular sin, he may be forgiven up to the third time, but not beyond that (Yoma 86b).
Before Yom Kippur, a chassid came to the chassidic master, Rabbi Bunim of Pshis'cha. The master reprimanded him for being remiss in the proper observance of a mitzvah and the man promised that he would be more diligent - but the following year, the same scene was repeated.
When the chassid again asserted that he would mend his ways, the Rabbi invited him to a game of chess. During the game, the Rabbi intentionally made a wrong move and asked permission to be permitted to retract the move. "You know the rule, Rabbi," the chassid said, "once you have removed your hand from the piece, the move is final." Nevertheless, he gave in. Later in the match the same thing happened, and the man said, "I am sorry, Rabbi, but you cannot keep on retracting moves. You must think before you move; once you have done so, it is final."
"Exactly, my son," the Rabbi said, "and if this is so a game, how much more so in the serious business of life."
Just as there cannot be endless retractions in chess, so we must realize that some actions are final. Repeating the same sin after one knows it was wrong indicates an attitude of carelessness.