Nicely put. I often consider this problem when I think counterfactually: "What would have happened if I missed the train? Where would I be right now if I hadn't lost my pencil two days ago?"
I have an instinct to just dismiss this kind of musing, because I think to myself that even the smallest changes propagate in wildly unpredictable ways, with potentially vast consequences -- think "the butterfly effect"-- and so it's pointless to even try to reason counterfactually.
But of course it's not pointless, and our minds do this naturally all the time: holding everything else equal, we're constantly twisting this knob and that one, imagining all kinds of possible worlds, exploring the space of what could have been.