Keep Your Cool
Explanation: "Cool" is described as composure and the absence of excitement in a person. To maintain composure, you wouldn't get excited and you would act normally.
Meaning/Usage: To act normal
"I know you're upset but you have to keep your cool."
"Here she comes, keep your cool."
"Keep your cool. You don't want to mess this up."
A. "Are you ready for your job interview?"
B. "Yes, but I'm a bit nervous about it."
A. "Just keep your cool and you will do fine."
Other Common Sentences
"You need to stay calm."
"Don't let it bother you."