I like to watch flowing water. What you will notice from time to time is an unexpected “bump” of water. This is a feature of a chaotic system: the periodic outlier. The system itself, however, is defined by the arithmetic average, the statistical behavior, over time. The system may never for a moment hit these abstractions perfectly, but will hit them approximately all day long.
People are defined by what they do most of the time, over time, and why. A single sin does not alter a personality, necessarily, except to the extent that either guilt or the rationalization implemented to ease guilt affect qualitative change in how that person moves through the world. This would be the equivalent of permanently redirecting the water.
It seems to me too that guilt becomes “stuck” when you fail to achieve the growth being asked of you. When you do something you feel really bad about, you do not have a choice of returning to the status quo ante. You either grow or you diminish. You either permanently internalize a new personality gestalt, or you downgrade to the sort of person who can make the incongruities evoked go away by simply ignoring previously dear principles; by in effect rejecting who you were and had to that point chosen to be.