The opposite regret is quieter and lives longer. You left. The decision felt clean at the time, or at least cleaner than staying. Then a year passed, or five, and now you are looking back at a relationship that you can almost see clearly, and the question that does not let you sleep is whether you walked out on something that might have become the thing you have been searching for ever since. The fear is not that you ruined their life. The fear is that you ruined your own and have spent the years since chasing a feeling you already had.
This regret is particularly cruel because it is unfalsifiable. You cannot run the experiment. You cannot rewind to the apartment in the autumn of that year and choose to stay one more season, and one more, and watch what the relationship became. You can only run the simulation inside your head, and the simulation is being run by a brain whose current loneliness will tilt every variable toward a fantasy that the staying would have worked. The simulation is not data. It is a wish wearing the costume of memory.
The other reason this regret persists is that leaving is asymmetric to staying in how it shows up later. When you stay too long, the cost is visible, accumulating, undeniable. When you leave too soon, the cost is a counterfactual, a road not taken, and counterfactuals are infinitely flexible. The person you left can be made, in retrospect, into anyone you currently need them to have been. The relationship can be made into anything you currently need a relationship to be. You are no longer remembering them. You are auditioning them for a part in your present loneliness.
What was true at the time of leaving was usually some combination of: a feeling you could not name but that was loud, a pattern that had not yet broken but showed no signs of breaking, a version of yourself that was disappearing, a future you could see down the corridor of years and did not want. These are not small reasons. They are the reasons human beings have always left, and they are the reasons your particular self, with your particular history, was right to take seriously. The leaving was not a glitch in your judgment. It was your judgment.
The Law of Humility (Law 0) cuts both ways here too. It tells you that you do not have access to what would have happened. The road not taken is not a road you can survey from where you stand. It is a fog you are choosing to populate with the shapes of your current need. To insist that staying would have produced the love you now lack is to claim the kind of certainty about counterfactuals that no one has ever earned. The honest position is: I do not know. Staying might have produced everything. It might have produced a slow ruin. Both versions live in the fog. The fog is the fog.
There is a specific shape this regret takes around the question of children, around the question of family, around the question of the life you can no longer easily build at your current age. These regrets are not invented. The constraints are real, and the grief of those constraints is real, and the fact that an earlier different choice might have changed them is also real. But the grief of constraint and the regret of decision are not the same thing. You can mourn the life you do not have without convicting the past self who made the decisions that led here. She did not know about the constraints either. She was making choices inside her own fog, with her own information, and the information she had was telling her loudly that the road she walked away from was not the road.
What you owe yourself is the same thing you owe yourself in the other direction: the end of the retrial. You owe yourself the honest acknowledgment that you might have been wrong, that the simulation cannot resolve it, and that the only place freedom is available is in the present, where the actual question is not "should I have stayed" but "what do I do now with the self who left." That self is still here. She is still capable of love. She has more information than she did. She is allowed to use it without first paying off a debt she cannot, by the structure of time, ever repay.