This is a question I've had for some time.
The answer comes from understanding the difference between our justification and sanctification/ongoing fellowship with God.
When we come to faith in Christ, all of our sins are legally forgiven. We don't need to continually pray for forgiveness from sins in a salvation sort of way.
Instead, when Jesus tells us to pray "Forgive our sins" in the Lord's Prayer ("our daily bread" implies we are to be praying the Lord's Prayer daily, which includes forgiveness of sins), He seems to be meaning forgiveness in the sense of how our daily sins disrupt our fellowship with God, displease Him or are bringing on Fatherly discipline.
"Restore to me the joy of your salvation." (Psalm 51:12)
"Do not grieve the Holy Spirit." (Ephesians 4:30)
"If our heart does not condemn us, we have confidence before God." (1 John 3:21).
We need to confess our sins to God (and at times to other believers when the sin has become more powerful in our life) after coming to Christ not to re-attain our salvation, but to re-gain our intimate fellowship with the Lord.