Hi there, it's an interesting and challenging topic, thank you.
Initially, when we repented from our sins as unbelievers, by the Gospel messages ministered unto us apparently. We were born again able to 'see' the Kingdom of GOD through these messages. Moreover, even further born of water and of the Holy Spirit becoming the new creation spirit man, in order to 'enter' the Kingdom of GOD.
Now, as many as received Jesus, to them gave Jesus the power to become sons of GOD, even to them that believe on Jesus' name. So, we were forgiven from all our past sins and our body cleansed completely into a new vessel. A new vessel then disciple in the New Testament of Lord Jesus Christ, to be filled as 'doers' and 'abiders' to every words/teaching of His, even through the Apostles. And, as well disciple in the Old Testament, to be as references and historical facts and evidences to the Kingdom of GOD in the past.
Therefore, as the true born again church, we henceforth fall short in being 'doers' and 'abiders' to Jesus' words/teaching in the New Testament. Apparently, this short comings of ours are counted as 'lawlessness', and Apostle John depict this 'lawlessness' as 'sin'. This short comings happens due to being in the flesh, where it's 'passions' and 'desires', it's 'fruits', are evident in us. Where Apostle Paul mentions, that these 'fruits', we supposedly should have had them crucified already. (Galatians 5:19&24&26)
Conclusion, that's why 1John1:9, is given to all true born again believers, to confess their sins based on these short comings. Where those who fail as 'doers' and 'abiders' to the words/teaching of Lord Jesus Christ in the New Testament. Therefore, in order for them to stop sinning and sin no more, progressively in their lives, and found pure upon His return.
In the Gospel the Lord have already said, "Be ye perfect as your FATHER in Heaven is perfect." And, "Your sins are forgiven, sin no more, or something worst shall happen to you."