How to think straight when scared?

Well you know how theres some people that think so clear and crisp when there scared like there not even scared right? yeah well thats not me..... so last night i layed down and i saw the closet door opening ever so slowley i started panicking and going ""what do i do?! what do i do?!"" i was so scared until i figured out my eyes were playing tricks on me cuase of the shadowed casted bu the closet cuase of my computer light so after that whole event was over i was like "" if i was scared why didnt i grab my knife i hide in my matress its like half of a arm lengths away? so anyways how can i think straigh when scared ?????