Create in me a clean heart, O God, and renew a right spirit within me.
Our problem runs deep. We need more than forgiveness of sinful acts. We need our heart to be transformed but we are powerless to change it ourselves. Maybe we can disicpline ourselves for a while to modify certain behaviour, but if the sinful desires remain in the heart then it's only a matter of time until we fail. We need God to change our heart. The good news is that he can and will.
Who can say, “I have made my heart pure; I am clean from my sin”?
No one. But...
I will give you a new heart, and a new spirit I will put within you. And I will remove the heart of stone from your flesh and give you a heart of flesh. And I will put my Spirit within you, and cause you to walk in my statutes and be careful to obey my rules.