When we come to Christ, He takes away all our sins. But sin is like a virus that still resides within us, and we all still sin -- even if we aren't aware of it. The Bible says, "If we claim to be without sin, we deceive ourselves and the truth is not in us" (1 John 1:8).
The first step is to realize that sin isn't just something we do (like lying, stealing or committing immorality). It also involves our inner motives and thoughts -- things like selfishness, impure thoughts, jealousy and envy. And those are just as serious as our outward sins -- in fact more so, because they cause our outward actions. Jesus warned, "From within, out of men's hearts, come evil thoughts, sexual immorality, theft, murder, adultery" (Mark 7:21).
But we sin not only by what we do, but by what we fail to do. Have you ignored someone who needs a kind word or some practical help? Have you forgotten to pray for people who need your prayers? God is just as concerned about the good things we fail to do, as He is about the things we shouldn't do.
The psalmist prayed, "Search me, O God, and know my heart; test me and know my anxious thoughts. See if there is any offensive way in me" (Psalm 139:23-24). Make that your prayer -- and then thank Him for His forgiveness in Christ.