It’s odd that “all in your head” means “not a serious problem” or “you can fix it by yourself”. It seems to me that a problem being located in the head/brain/m

It’s odd that “all in your head” means “not a serious problem” or “you can fix it by yourself”. It seems to me that a problem being located in the head/brain/mind is, almost by definition, particularly difficult to fix, let alone to fix without external guidance.