Be all in somebody’s/the mind
Definition 1 of 1
Pro Tip 1/3Perception Bias
This phrase indicates a person's belief or fear that has no basis in reality, showing the power of perception over facts. 
You think everyone dislikes your idea, but that's all in your mind.
Pro Tip 2/3Subjective Reality
It suggests that what someone is experiencing is subjective and may not be shared by others. 
She felt ignored by the group, but it was all in her mind.
Pro Tip 3/3Reassuring Phrase
Used to comfort someone when they are worried about something that is not true or real. 
Don't worry about the noises at night; it’s all in your mind.