Remembering that horrible feeling of being in an exam and having a mind-blank, realising you just don't know what you need to know. Then remembering the feeling of coming out of an exam that you're really confident about.
Also, thinking about how everyone else is working and I'm not and if I don't work I won't get into the university I want to. In addition to disappointment, there's the embarrassment of not achieving what I know I can achieve.