Here's the first seven verses:
Have mercy on me, O God, according to your steadfast love; according to your abundant mercy blot out my transgressions.
Wash me thoroughly from my iniquity, and cleanse me from my sin.
For I know my transgressions, and my sin is ever before me.
Against you, you alone, have I sinned, and done what is evil in your sight, so that you are justified in your sentence and blameless when you pass judgment.
Indeed, I was born guilty, a sinner when my mother conceived me.
You desire truth in the inward being; therefore teach me wisdom in my secret heart.
Purge me with hyssop, and I shall be clean; wash me, and I shall be whiter than snow.
Although most of the psalms are communal prayers, this psalm is expressed as an individual cry, Have mercy on me, blot out my transgressions, wash me, and purify me.
And I can ask this of God because mercy is what God is like, because mercy befits God's faithfulness, because God is abundantly compassionate.
Sometimes, we need reminders that we are sinning. And, sometimes, we are so burdened by our sins that we need reminders that God is compassionate.