The phrase "prey upon the mind" refers to a situation where someone's thoughts or emotions are constantly consumed or disturbed by a particular issue or worry. There are a number of different synonyms that could be used to convey this idea, including "haunt," "plague," "torment," "disturb," "obsess," and "preoccupy." Each of these words suggests a persistent and often troubling concern that dominates a person's thoughts and emotions. It's important to address and alleviate such mental preys in order to find peace and focus on more positive and constructive aspects of life.