“There is no soundness in my flesh because of Your anger, Nor any health in my bones because of my sin. For my iniquities have gone over my head; like a heavy burden they are too heavy for me. My wounds are foul and festering because of my foolishness.” (vs3-5)
It is our fault because of sin; we sinned in our father Adam and brought the curse upon ourselves, even slavery to sin and the misery thereof (vs1-5). He says, “Because of my sin... My wounds are foul and festering because of my foolishness”. We brought death, transgressions galore, and pain upon pain upon ourselves by our own refusal to obey. It is not Adam's fault, but you and I sinned. Romans 5:8, “through one man sin entered the world, and death through sin, and thus death spread to all men, because all sinned”. It isn't our parents fault, the bosses fault, or anyone else's fault that WE are sinners; it is ours alone. The things that we have done are not excused by what others did to us; the Lord Jesus, the Son of God, never sinned, though He was chased at His birth, raised by imperfect parents, hated by the world, forsaken by His friends, and made to bear the curse of God's hatred upon His body when He died for God's elect. He did no sin, and we can't blame anyone for the sin that we've done, except ourselves; it is not God's fault, either.
If you realize this, then you can and will obtain mercy, because only the blameable are forgiven, and only sinners meet with God in Christ (Luke 18:9-14).
Secondly, our sin causes us great misery and grief (vs6-10). He says, “I am feeble and severely broken; I groan because of the turmoil of my heart. Lord, all my desire is before You; and my sighing is not hidden from You. My heart pants, my strength fails me; as for the light of my eyes, it also has gone from me”. Not only are we to blame, but we've brought agony upon ourselves, separation from God, and the miserable consequences of sin; not only have I done evil, but evil has come upon me justly, even grief, woe, helplessness, and despairing thoughts of self. Sin isn't a friend; it is a foe, leading us to pain through the temptation of pleasure, causing us to hurt ourselves while promising, “It isn't so bad”, to get us to indulge. Satan told the woman, “You will not die. For God knows that in the day you eat of it your eyes will be opened, and you will be like God, knowing good and evil”, choosing for yourself what's right (Gen 3:4-5)! But, sin hurts, and by this pain, we seek after mercy and find God gracious to sinners.
Finally, we have utter confidence in the Lord (vs11-22). He says, “For in You, O Lord, I hope; You will hear, O Lord my God. For I said, 'Hear me, lest they rejoice over me, lest, when my foot slips, they exalt themselves against me”. He is merciful to sinners. The other day, I passed by a drunk holding up a, “Single Parent, Need Help, God Bless”, sign. He was in his late 50's, bore no evidence of children, and stumbled drunkenly while holding his sign. I did not give, but I kept driving. Yet, God is MERCIFUL to sinners, to people who ruined their lives, brought pain upon themselves, and who want to be helped. If you approach Him as you are – guilty and miserable – then He'll show you who He is – merciful, gracious, and wise - 100% of the time. The sins you hate have brought you to Christ, so that you may be forgiven, alleviating the pain of conscience and reconciling you to God. He will give strength to honor Him now, for Christ's sake, because He was made sin, guilty, and miserable for you, suffering the wrath you deserve in full, until it is put away from God forever! Pain is needful, guilt is preliminary to mercy, and God will assuredly hear. He says, “Do not forsake me, O Lord; O my God, be not far from me! Make haste to help me, O Lord, my salvation!” He is 100% my salvation when I've found myself, by confession and pain, to be 100% unworthy, hell-worthy, and sinful.