David’s heart condemned him

“And David's heart smote him after that he had numbered the people. And David said unto the LORD, I have sinned greatly in that I have done: and now, I beseech thee, O LORD, take away the iniquity of thy servant; for I have done very foolishly.” - 2 Samuel 24v10

David never did grow into perfection. Even now, at the very end of his life, he had a few problems with trusting God completely. God told David to trust him and not worry about his numbers, but David went ahead and called for a census anyway.

Immediately his heart was smitten. He knew he had acted foolishly and sinned against god by not trusting Him. He made no excuses; he just admitted his sin and foolishness. One thing we can admire about David is that when he was wrong he admitted it, he never tried to make any kind of excuse. That sets an example for us when we are guilty of sin.
There is even a better blessing. David did not know the full forgiveness offered by Christ. He had to deal with his guilt, for there was no Holy Spirit to removed the sin and the guilt. In Christ we have the blessing of sweet forgiveness and there is no need for our heart to condemn us as David’s did. We must repent and forsake our sin, but also know that the blood of Jesus Christ covers our sins, and our hearts can no longer condemn us. John 3v20-21 gives us this blessed knowledge, “For if our heart condemn us, God is greater than our heart, and knoweth all things. Beloved, if our heart condemn us not, then have we confidence toward God.”