Maimonides explains the difficulties of admitting by ourselves that we have sinned. Taking charge of our misdeeds -hakarat haChet- 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, and evaluate objectively and responsibly our past actions. Objectivity, goes against our normal emotional behavior, which pushes us to justify our actions, acting as our own advocates. Thus, whatever bad things we do become "good in our won eyes" (4:2).
David haMelekh committed a terrible sin, when he took bat-sheba, a married woman and sent her husband to the battlefront. David, did not repent by the call of his own conscience. Natan, the prophet, was sent by God to admonish David and help him realize the seriousness of the sin he committed. Natan presented David, who in his capacity as a King was also the supreme Judge of Israel, with a (fictitious) case: A rich man owned thousands of animals. His neighbor, very poor, had only one lamb. One day, the rich man received a guest. In order to save one of his own sheep, the rich man decided to steal and slaughter his neighbor's lamb, which he dearly loved. As Natan had expected, the King reacted angrily. 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 with the sentence he issued as a judge 'against himself', David recognized his wrongdoing, repented and admitted: 'chattati laHashem...' , 'I have sinned against God'.
For these transgressions, David was not permitted to build thebet-haMiqdash, but God accepted his Teshuba. Natan opened the King's eyes helping him to realize the gravity of his actions objectively, allowing him to free himself from the psychological mechanisms of self-defense which represent one of the biggest obstacles on the way to repentance.