it's all in your mind
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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.
Farlex Dictionary of Idioms. © 2022 Farlex, Inc, all rights reserved.
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.’”
The Dictionary of Clichés by Christine Ammer Copyright © 2013 by Christine Ammer