The Repentant Heart is a Stained Heart
This is the lowest point in David's life. The repentant heart is a stained heart. God opens our eyes so that we see our sin. We try all kinds of things to get rid of the stain but we can't. Instead we turn to God to do it. David mentions hyssop which was dipped in the blood of a lamb to cover sin in the Old Testament. David knew the only way for sin to be dealt with is for blood to be shed. The next time we see hyssop is as Jesus is on the cross and is given wine from a sponge on hyssop. At the cross all of David's guilt and ours was poured upon Him.