AFAIK, according to the counterpart theory, it is true of me that I could have lived a different life, if my counterpart in another possible world did live a different life. But where is it true that I have counterparts? It seems not to be true in just one world, but how (according to the background modal realism) can it be true in multiple worlds? Or is this one of those cases where there is an important difference between true-at-a-world and true-in-a-world?
(My objection/confusion might be framed in terms of the phrase "possible counterpart." Again AFAIK, no object in the actual world is my counterpart except (weirdly) myself. So no other object in the actual world is my counterpart in some other possible world, obviously. But then how would we make sense of the phrase? Are my counterparts necessarily my counterparts? But they're not my counterparts in every possible world besides in the sense that all of them together are; but doesn't the background theory have it that the arbitrary mereological sums spanning the plurality of worlds, though they exist, perforce lack actuality?)