How can I persuade my sister to regain her confidence?

This is a psychological effect. If you believe it, your grades will definitely be bad, because you have no confidence. If you don't believe it, it doesn't matter. Look at your sister, you should have believed it. You can fly over and tell her a heroic story of being physically disabled and determined, and then tell her that even so, we must overcome our destiny and take control of our future, or you can do the math again. What grade is she in primary school? People don't know the future.