be all in (one's)/the mind

To exist solely in one's thoughts, rather than in reality. Don't tell me that my worries are all in my mind—you know this can be a dangerous area. Your presentation sounded great, honestly. Your fears are all in the mind. Cindy isn't mad at you, it's all in your mind.

it's all in your mind

You’re imagining it, it’s not real. This cliché differs from such earlier uses as in my mind or to my mind, which meant “in my opinion” from the early 1500s but are heard less often today, and on one’s mind, meaning “occupying one’s thoughts,” dating from the mid-1800s. The current cliché is generally used as a denial of another’s statement, as in “‘I’m sure this woman doesn’t like me.’ ‘No, it’s all in your mind.’”