My sensei said, when we are worried about something big like exam, don’t be. Just ダメもと (dame moto).
Dame moto is short form of ダメでもともと (dame de moto moto) which means nothing to lose.
Even if you fail the exam, doesn’t mean our ability decrease. We just go back to square one or our original state.
When we never challenge ourselves, how can we tell we are growing?