This is a common question and shows how mental fatigue is commonly taken care of today. However this is a misunderstood concept. While self talk can have an improvement in mental fatigue levels and RPE (rate of perceived exertion) it requires mental capacity to carry out. Furthermore, when cognitive fatigue really kicks in, an athlete will slow down and reduce performance unconsciously. The brain decides to hit the brakes in order to protect the body from the mounting levels of fatigue. Therefore it is not possible to talk yourself out of something your body is doing almost against your will.