It's like your teachers said. Fantasy makes your dreams less likely to come true.
Positive fantasies "'make energy seem unnecessary' say Heather Kappes and Gabriele Oettingen. 'By allowing people to consummate a desired future,' the researchers explain, 'positive fantasies trigger the relaxation that would normally accompany actual achievement, rather than marshaling the energy needed to obtain it.'"
But only positive fantasies. If your daydreams feature worrying that you'll never achieve your goals, that's supposed to energize you. According to this research, pessimists should be more successful than optimists, no?
"Immersion in the life of the world, a willingness to be inhabited by and to speak for others, including those beyond the realm of the human, these are the practices not just of the bodhisattva but of the writer." --Jane Hirshfield