Because the mind is not ready. The mind does not believe, is not convinced. In the same way as the body need to be trained also the mind need training. If the goal being too high the mind will not accept it if it is not very fit and strong. The way to overcome this is, 1. Muscular relaxation, 2. Mind relaxation and 3. Mind development/reprogramming/taking control over ones own mind.

Via Robert Axelsson