Rather than directing us toward Christ by his typological righteousness (as is the case in Pss 1-24), David is here shown to be a sinner needing forgiveness.  He recounts how he experienced God’s fatherly displeasure.  This was because he was consciously harboring sins.  But after confessing them in faith and being renewed in his justification, he was overcome with joy.  This compelled David to encourage us to do likewise – not to be stubborn like the horse and mule but to humbly confess in faith, in order that we may rejoice in our standing before God.