Summary: David talks through God's forgiveness. Without it, he wastes away. Once he has it, he is safe and hears praises to God from others who have it. And he gets to make those praises himself!
Response: If I remember correctly, the selah thing here is a break in the action. If this had had musical accompaniment at any point, I'm pretty sure the guitar (or more likely lute) solo would have gone there. But it helps chop the song up so we get these different aspects of the story, which I greatly appreciate. And we have yet another glimpse of Human David, who is less than perfect and will admit it! I like that guy because I know that I'm that way, too.