fight with a gorilla

Fighting with a gorilla is what you can say when you are having an internal conflict and/or are dealing with something you wish to keep secret from others in the conversation.

"Paula's really been fighting with the gorilla lately! Have you heard about the big break up? Tough situation all around."
or
"Y'know I would go out tonight but I've just been having a fight with a gorilla all day today. Maybe I'll talk to you about it later. Go have fun without me."