We sin. Then we cover our sin. Then we lie to ourselves and tell ourselves we haven't really sinned. As if such behavior wasn't bad enough, we then repeat the process over and over again - every day.
Occasionally, we say, think, or do things we consider to be really bad, and then when we hide those sins from ourselves or others, we feel even worse. The result of such suppression is a growing load of guilt, a load that, if left unconfessed, begins to result in the physical symptoms the Psalmist mentions in this psalm. He says, "There is no soundness in my flesh because of Your anger, nor any health in my bones because of my sin."
David had experienced all that. His sin with Bathsheba was pleasurable for the moment - but it added an unbearable weight to his load of guilt, especially after he tried to cover it with the subsequent murder of her husband. He personally experienced all the consequences he lists in this psalm.
There was eventual confession and forgiveness for David, though, and his love for God and God's people then moved him to lead the people in this song - so that they might not have to experience the same consequences of guilt he had experienced. Hopefully, as he led them in these words, they'd see their need for daily confession, their need to remain guilt free and open to their God.
For us, too, there's need for daily confession and forgiveness. Read carefully here about the result of not confessing sin, of attempting to hide it and of pretending it never existed. Then, when you've done that, consider carefully David's eventual explosion in prayer. He finally has to cry out, "For I will declare my iniquity; I will be in anguish over my sin . . . Do not forsake me, O Lord; O my God, be not far from me! Make haste to help me, O Lord of my salvation!"
The lesson is that we shouldn't procrastinate, as David sadly did, to confess every sin. After all, Jesus Christ knew all your sin long ago, and so you won't surprise Him with any of it now. He knew it when He became your Sponsor and when He "gave Himself, the just for the unjust, that He might bring us to God." He knows you and still loves you! And the result of your Savior's love is expressed by John. He says, "If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness!"