John Piper made some comments on this Psalm in a recent sermon that are worth quoting at length. He explains some of the darker feelings I have been having. Here is part of what he says, and I encourage you to follow the link and read the whole thing.
…David is forgiven for his sin and justified in the presence of God. But what Psalm 51 describes is what David felt and thought as he laid hold on God’s mercy. Some might say that Christians after the death of Jesus do not pray and confess this way. They should not think and feel this way. I don’t think that’s right…
This is foundational to everything. Being a Christian means being broken and contrite. Don’t make the mistake of thinking you get beyond this in this life. It marks the life of God’s happy children till they die. We are broken and contrite all the way home—unless sin gets the proud upper hand. Being broken and contrite is not against joy and praise and witness. It’s the flavor of Christian joy and praise and witness.
(John Piper, “A Broken and Contrite Heart God Will Not Despise,” as downloaded from here on today’s date.)
I know I am justified by what Christ has done for me. He has paid the penalty for my sins, and there is nothing I can add to or take away from His payment. But it is good to know that feelings of remorse, and even revulsion, are normal in view of my past sin. And, somehow, because I know these feelings are normal, I can turn myself over to God in a wholehearted way. I can think of myself rightly as God lets me feel His love once again.
Thanks be to God for His indescribable gift!