'Cognition' is a word used to denote a thought or belief, and 'restructuring' involves challenging a cognition by looking at the evidence for or against it. The basic idea behind cognitive restructuring is that thoughts themselves are not facts; you need to consider whether the thoughts you are having are true and to consider whether there are any other ways to thinking about a particular situation. At first this can be a real challenge: we often believe what we think and don't question our own thoughts. However, often our thinking is biased, and with perfectionism it can be biased towards thinking too negatively and harshly about yourself.