What should I say if asked, "Why can only I not do it?"

First receive the feeling—"you felt that way"—and listen without denying it or over-reassuring. Then respond concretely: "everyone has strengths and weaknesses," "you find ○○ hard, but you're good at △△." Explain in a way that doesn't blame them: "it's not your fault, and not because you didn't try hard enough—your brain just works a little differently." Adding what you can do together—"for the hard parts, this trick makes it easier"—helps them take it in positively rather than ending at "I can't."