I suspect that overthinking often involves leaving the present moment in order to speculate or assume about the future.

Perhaps there would be less overthinking if one could stay in the present. For example: you mentioned speculation. Yes, you could speculate about how T will change after her return from surgery. That sounds like a path to overthinking.

Or, you could instead say that right now your task is to get through the current moment, and you will worry about whether or not T has changed when you see her again.