If you commit a sin, then you might think you are a sinner. If you hold to that identity so assiduously that you keep on sinning because of it, that's a problem.
So we usually (partially or wholly) disown our past selves that sinned, say that we are a new person, which helps us avoid that trap.
There are two ways to commit a sin: to do something wrong, or to fail to do the thing you need to do. If you're worried about not doing the thing that you are supposed to do, you might watch out for things that bend your identity away from being the kind of person who would do them. But you may well find yourself intentionally or unintentionally doing something that moves your identity away from doing what you need to do.
If that happens, you might disown your past self that was not "a good worker" or "in tune with the truth" or whatever other identity enables you to do what needs to be done, just as you would disown your past self that committed some kind of infraction.
If you mess up or had a bad intention in the moment, then it's true that you were out of line with God's values. But what would really make you be out of line with God's values is if you stay there and never get back on track.