Long-term
You pull back. Stop initiating as much, take longer to reply, go quieter. Nothing dramatic. And suddenly she's more present than she's been in months. More attentive, more available, doing things she hadn't been doing. It's what you wanted. It arrived the moment you stopped wanting it.
The game
While you're fully present, she has no signal that anything is at risk. The cost of changing is real (effort, adjustment, admitting that something needs to change) and the cost of not changing is zero, because you're still there.
When you pull back, the information updates. Now not changing has a cost: losing you. The behavior that wasn't worth the effort last month is worth it now. Nothing changed about what she's capable of. What changed is the stakes.
The equilibrium
Full presence keeps the stakes low and minimum viable behavior stable. A credible signal that you're leaving makes maximum effort rational. The relationship has two equilibria, and which one you're in depends on whether she thinks you're actually going.
The exit interview begins when you give notice, not when you leave.
Dominated strategies
Reading the change as the problem being solved. The change is real — she's doing the things you asked for, and that's not nothing. But it's a response to the threat being present. Go back to full presence, remove the threat, and you're testing whether it holds without the pressure that produced it.
The change tells you what she's capable of. It doesn't tell you what she'll maintain once you're no longer on your way out. Those are different questions, and conflating them is what sends people back into the same dynamic with fresh optimism.