David's sin had been revealed and he had repented but the damage and the effects that others felt because of his sin would be deep scars upon their souls. Repentance through the blood of Christ. Once we admit that we have sin and say I'm sorry asking for the Lords forgiveness you no longer have to obey the evil prompting of the world and of the flesh. You will not only be free of sin but free to obey God and walk in righteousness and purity. The bible says that we are all born into sin.
Sin is our nature.
Lust is natural for the sinful person. This sin will one day result in a spiritual and eternal death. But Christ who became sinful flesh upon the cross who shed his blood as a final sacrifice to sin has invited and called us all to repentance. So you say, I can't control the thoughts that come into my head. Once the thoughts get in how do I get them out.