Interesting point you brought up. Some people seem to argue that John is focusing on confessing every single sin that we commit as we commit them (keep a specific inventory) as an additional requirement to remain cleansed (1John 1:9) and if we forget a sin, we're toast!
Others would argue that John instead has in mind here a settled recognition and ongoing acknowledgment that one is a sinner in need of cleansing and forgiveness.
*Notice that verse 8 says, "If we say that we have no sin, we deceive ourselves, and the truth is not in us." Verse 10 says, "If we say that we have not sinned, we make Him a liar, and His word is not in us" and this is in CONTRAST TO - “if we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.”
Others would argue that John instead has in mind here a settled recognition and ongoing acknowledgment that one is a sinner in need of cleansing and forgiveness.
*Notice that verse 8 says, "If we say that we have no sin, we deceive ourselves, and the truth is not in us." Verse 10 says, "If we say that we have not sinned, we make Him a liar, and His word is not in us" and this is in CONTRAST TO - “if we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.”
Isa. 1:18 "Come now, and let us reason together, saith the LORD: though your sins be as scarlet, they shall be as white as snow; though they be red like crimson, they shall be as wool.
Isa. 43:25, 26 "I, even I, am he that blotteth out thy transgressions for mine own sake, and will not remember thy sins. Put me in remembrance: let us plead together: declare thou, that thou mayest be justified."
If God is willing to call us clean, shouldn't we be also?