Again, the worst sin is not some outburst of gross transgression, forming an exception to the ordinary tenor of a life, bad and dismal as such a sin is; but the worst and most fatal are the small continuous vices, which root underground and honeycomb the soul. ~Alexander MacLaren, David's Cry for Purity, Ps 51:10

Another very solemn and terrible thought of what sin is, lies in that final word for it, which means 'missing an aim.' How strikingly that puts a truth which siren voices are constantly trying to sing us out of believing! Every sin is a blunder as well as a crime. And that for two reasons, because, first, God has made us for Himself, and to take anything besides for our life's end or our heart's portion is to divert ourselves from our true destiny; and because, second, that being so, every attempt to win satisfaction or delight by such a course is and must be a failure. Sin misses the aim if we think of our proper destination. Sin misses its own aim of happiness. A man never gets what he hoped for by doing wrong, or, if he seem to do so, he gets something more that spoils it all. He pursues after the fleeing form that seems so fair, and when he reaches her side, and lifts her veil, eager to embrace the tempter, a hideous skeleton grins and gibbers at him. The siren voices sing to you from the smiling island, and their white arms and golden harps and the flowery grass draw you from the wet boat and the weary oar; but when a man lands he sees the fair form end in a slimy fish, and she slays him and gnaws his bones. 'He knows not that the dead are there, and that her guests are in the depths of hell.' Yes! every sin is a mistake, and the epitaph for the sinner is 'Thou fool!'  ~Alexander MacLaren, David's Cry for Pardon, Ps 51:1, 2

Chambers on Repentance

Repentance Godly sorrow produces repentance leading to salvation. 
—2 Corinthians 7:10 Conviction of sin is best described in the words: My sins, my sins, my Savior,
How sad on Thee they fall. Conviction of sin is one of the most uncommon things that ever happens to a person. It is the beginning of an understanding of God. Jesus Christ said that when the Holy Spirit came He would convict people of sin (John 16:8 ). And when the Holy Spirit stirs a person’s conscience and brings him into the presence of God, it is not that person’s relationship with others that bothers him but his relationship with God— "Against You, You only, have I sinned, and done this evil in your sight . . ." (Psalm 51:4 ). The wonders of conviction of sin, forgiveness, and holiness are so interwoven that it is only the forgiven person who is truly holy. He proves he is forgiven by being the opposite of what he was previously, by the grace of God. Repentance always brings a person to the point of saying, "I have sinned." The surest sign that God is at work in his life is when he says that and means it. Anything less is simply sorrow for having made foolish mistakes— a reflex action caused by self-disgust. ~Oswald Chambers, My Utmost for His Highest, December 7

Confession

A child of God will confess sin in particular; an unsound Christian will confess sin by wholesale, he will acknowledge that he is a sinner in general; whereas David doth, as it were, point with his finger to the sore (Ps. 51: 4): "I have done this evil"; he doth not say, I have done evil, but this evil. ~Thomas Watson, Gleanings

Matthew Henry on Sin

This should greatly humble us for all our sins, that they have been committed under the eye of God, which argues either a disbelief of his omniscience or a contempt of his justice. ~Matthew Henry, Commentary, Psalm 51

Matthew Henry on Sin

Sin defiles us, renders us odious in the sight of the holy God, and uneasy to ourselves; it unfits us for communion with God in grace or glory. When God pardons sin he cleanses us from it, so that we become acceptable to him, easy to ourselves, and have liberty of access to him. ~Matthew Henry, Commentary, Psalm 51