She ended it, or you ended it because she'd already checked out, and something broke loose. You started going to the gym. Read more, slept better, made more money, got sharper. Six months later you're a different person.
Here's what's strange: the people who motivate that kind of change rarely get to see it. By the time you've improved, you've moved on.
The game
Her move was the exit. Her implicit assumption was that you'd remain available, that the door could be reopened later if she wanted it. She priced in the version of you that existed when she left.
Your move was improvement, ostensibly to win her back. But improvement takes time, and the market updates continuously. As your signals change, other women update their evaluation of you. You start getting attention you didn't get before.
Two things happen in parallel: she becomes more interested in you again, and your options expand. By the time you've improved enough to attract her back, you have enough alternatives that you're no longer anchored to her specifically.
She miscalculated the timeline. The window when you're improved enough for her to want you back but still interested enough to return is narrow. Act too early, you haven't changed yet. Wait too long, the window closes.
The equilibrium
The dynamic is self-defeating for whoever initiated it. She declined, expecting availability. The decline created the incentive to improve. The improvement removed the availability she was counting on.
The person who motivated the improvement rarely benefits from it, because the improvement changes the calculus that made returning attractive in the first place.
Dominated strategies
Improving with her specifically in mind.
If you're improving for her, you keep in contact to signal progress. You measure your growth against what she'd want. You stay anchored to someone who already declined, which signals you don't have better options yet, which updates her assessment downward. And you stop pushing the moment the relationship resolves. The improvement caps out exactly when the emotional trigger does.
The window when you've improved enough for her to want you back but still want her back is narrow. The improvement that was supposed to close the gap is what closes it.