I swore that I would commit seppuku if I didn’t get into Nankai. What should I do if I really didn’t get into Nankai?

As long as you take the mental exam, since you didn’t get admitted to Nankai, you can calm down a little and don’t care about the oaths you made, or go to Yihao University and study hard. If you want to, just repeat the exam and take the exam. A better school will treat your life as a child's play, not just for a joke, so that others will say whether you are stupid or not. So you still have to make the right choice rationally and lead your life on the right track.