Can I say "in my mind" instead of "in my head?"

Native speaker’s answer
Rebecca
I wouldn't suggest using "in my mind" instead of "in my head!" That's because the phrase referenced here is "it's all in my head," which suggests that you made it all up - you imagined it. "In my mind" wouldn't quite capture the same nuance. "In my mind" would suggest more of an opinion or idea. Ex: I made up a whole scenario in my head of what could happen. But it didn't. Ex: There are no ghosts here. It's all in your head.