Yisro told Moshe that he shouldn’t be the sole judge for the entire nation. There should be other judges helping him. The sefarim teach that Yisro’s point was valid. It was indeed impossible, according to the rules of nature, for Moshe to judge the nation alone. Yet, before Yisro asked that question, Moshe was able to judge the nation alone, because Hashem gives people strengths – beyond the rules of nature – when they want to do His will.
However, after Yisro asked his question, he brought the matter down to the realm of nature, and then it became impossible for Moshe to judge them alone. Now he needed aid. Therefore, Moshe sent Yisro home; his questions were weakening his strength. ~ R’ Elimelech Biderman Shlita
It is for the same reason that we fear something before it happens and cant imagine being able to deal with it. Because in reality, we truly would not be able to deal with it. But if that thing we fear actually happens, Hashem gives us the strength to pull through!! So there is really no point in worrying about something before it happens. And if/when it does, there certainly is no point worrying then either!