A few words from a book I own (Jiu-Jitsu University):
Everyone has fears, and every beginning jiu-jitsu practitioner at every academy will be accompanied by those fears when first stepping onto the mats. It may be the nightmare of claustrophobia, pain, or suffocation. In each situation, the student defeats these fears by facing them over and over again. At any moment, the student can tap to signal an end to the fight and start again and give it another try, over time he gradually liberates himself from the cycle of fear. This is a natural process of mental conditioning and it is the law for every student. Through this evolution, the grappler not only overcomes fear but also the instinctive tendencies to fight and waste energy. In place of his fear he develops confidence, fortitude, and peace in stressful times. In other words, he liberates himself from insecurity.