[be] all in {one's} head

to think or feel something in your mind, even though it might not be real or happening in the outside world

She constantly worries about what others think of her, but it's all in her head; people are generally supportive and accepting.
He often imagines worst-case scenarios, but it's all in his head; reality is rarely as dire as his thoughts.
The anxiety she feels in social situations is all in her head; others perceive her as confident and engaging.
He believes he's not good enough despite his accomplishments, but it's all in his head; he is highly skilled and talented.
The fear of failure that held her back was all in her head; she finally won the tournament.