Maimonides explains the difficulties of admitting to ourselves that we have sinned. Taking charge of our misdeeds -hakarat haHet- is probably the hardest step in the process of Teshuba. In the days before Rosh haShana we are expected to act as our own judges evaluating objectively our past actions. "Objectivity" is not an easy state of mind. It goes against our normal mental behavior. We naturally tend to justify our actions. Everything we have done becomes right in our own eyes.
David haMelekh committed a terrible sin. He took bat-sheba, a married woman, and sent her husband, a soldier, to a sure death in the battlefront. David did not repent by the call of his own conscience. Natan the prophet was sent by God to admonish the King and help him realize the seriousness of the sins he committed. Natan came to the court of King David (back then, the King was also the supreme Judge) and presented him with a fictitious case to judge: A rich man owned thousands of animals. His poor neighbor had only one lamb. One day the rich man had an important guest coming to his house. In order to spare one of his own animals the rich man decided to slaughter his neighbor's lamb, which he dearly loved. As Natan expected the King reacted angrily toward this injustice. King David said: "That man (=the rich guy) deserves to die!" Natan the prophet then turned to David and said: atta ha-ish.... "You are that man!". Faced now with the objective facts and having issued the harsh sentence against himself King David recognized his wrongdoing. He repented and humbly confessed: 'hattati laHashem...' , 'I have sinned against God'.
Due to the irreparable damage caused by these transgressions, David was not permitted to build the bet-haMiqdash. But God accepted his repentance.
Natan opened the King's eyes. And helped him realize the gravity of his actions objectively. By presenting him a fictitious case he forced the King to play the role of an impartial judge. Allowing him to "watch at himself from the balcony". Freeing him from the psychological mechanisms of self-defense. Which often represent the biggest obstacle in the way to repentance.
Rosh haShana and the Chinese Bamboo