I said, LORD, be merciful unto me: heal my soul; for I have sinned against thee.
I said, Lord, be merciful unto me. The "I" is emphatic in the Hebrew - i:e., What I "said" in general of the mercy promised in distress to those who are merciful to others in their distress, applies to ME: it is I who thereby claim thy promise, "Lord be merciful unto me."
Heal my soul, for I have sinned against thee. David, the type, rightly regards sufferings as designed by God to bring sins to remembrance, that the sufferer may confess his guiltiness, and seek healing of soul, as the necessary preliminary to temporal healing. The sufferings of the Antitype, Christ, were the penalty of our sins, which He appropriated as our Sin-bearer.