1)
Why does it say "Ani Amarti"?
1.
Radak: Until now, the one who visits the Choleh spoke. From now until verse 13, the Choleh himself speaks.
2)
Why does he request a cure only for his Nefesh?
1.
Radak: The physical illness is due to his sins. If Hashem will cure his Nefesh, i.e. atone for his sins, the body will be healed.
2.
Sha'arei Teshuvah 4:1: Illnesses of the Nefesh are bad Midos and sins. When the Rasha repents from them, Hashem heals his Nefesh.
3.
Malbim: He is at death's door. He ceased to pray for a cure of the body, for he despaired from life. He confesses, like one does before death, and requests a cure for the Nefesh - pardon of sins.
