As Yoda seeks to train Luke there is a famous sequence between both in which Luke reacts to the teaching with “I’ll try.” Yoda’s reaction “Do. Or do not. There is no try.” has become one of his well-known sayings.
But that may not be understandable without the context in which Yoda brought it up.
Luke was trusting what he had learned until then. It focused him on the things he believed could not be done. His imagination didn’t allow him to expand his ability to use the force to a challenge that felt too big to him.
What Yoda was telling him, was to use the force when he trusted himself to use it and to leave it aside when he didn’t trust himself using it. For his training it meant, that Luke had to let go of his beliefs about his ability.
He had to unlearn his sense of limitations and trust that he could expand his abilities. Trusting his limitations more than his abilities, Luke was sabotaging himself. He was keeping the limitations alive instead of expanding his abilities.