"Grant me, O Lord, according to Your kindness; just as David was likened to someone who had a wound in his hand and went to a doctor, asking him to heal him. The doctor replied that he could, but that the man's payment was too small. The man pleaded with the doctor, saying, 'Please take everything that I have and do the rest from your own [account], and show me kindness and mercy.' Similarly, David said to God, 'Grant me, O God, according to Your kindness; You are compassionate, and with Your great mercy, erase my transgressions.
You have done much kindness with me, and do so now, and erase my transgressions.' As it says in Psalms 17:7, 'Wonderfully show Your kindness, O Savior of those who seek refuge.' From You comes healing, and since the wound is great, give a great plaster. And he also says, 'Cleanse me from my sin.'
From here we learn that anyone who commits a sin is considered defiled in his soul, and he can only be purified with hyssop. And as it says, 'Purify me with hyssop, and I shall be clean.(Psalm 51:9)' Did David become impure with a grave sin? No, only with a sin that defiled his soul, as it says, 'My heart is defiled within me.' From here we learn that anyone who knows that he has sinned and prays for forgiveness, fears and shuns the sin, and discusses it with God, He forgives him.
But anyone who knows that he has sinned and deliberately repeats it, the Lord demands from him. As it says, 'Why should I fear in times of trouble, when the iniquity of my heels surrounds me?' (Psalm 49:6) Those who trample on sin with their feet, with their heels, the scorpion strikes them. He said to him, 'Your life is at stake.' So too, anyone who repeats his sin before God, He does not forgive him. Therefore, David said, 'My sin is ever before me.'"