When someone drops the physical body they do a review of their past life. Those who have led positive lives have the experience which is called heaven. Those whose lives are on balance negative experience the pain of their mistakes which is called hell. The learning is complete once these experiences have concluded. Then a future life reflects the learning of the past one.
If I am injured the pain consumes me and often seems that it lasts a long time when in fact the actual time is shorter. So when people think of heaven and hell being eternal, it's the subjective experience which seems eternal and not an objective eternity.
When a nasty, self-centered, manipulative person experiences the pain he's inflicted on others in magnified form and then also experiences the great joy of living a positive life in other lives, the person naturally will learn and improve over time.
So the mechanism is the past life review.
This would make sense if good and evil were just about learning and that's all that is needed to fix. But to me it's about being true to yourself and God. If you fail that the first time, the habits kick over and the lying nature increases. I don't think the review would make a soul more honest. They had better chances of getting it right when they had not failed yet.
I can tell myself, I don't always act to what I know. So knowledge does not mean you will be more true to it. Also, the mechanism is faulty in that we are born as babies. This means we forget past lives in this case. The ingrained nature would be from deeds, not review. The deeds that make us return in worse state and less sincere, would bound us to failure.
The first promise if we don't fulfill it, I don't see us ever being true to it after.