Watch so many that you become desensitized, or watch some G rated movies until you've washed the Horror out of your mind.
Watch a comedy.
You have to mentally accept that what you've seen is just a movie. Think about everything that happened today. Think about other things in your life. Snap yourself back to reality. And watching some cartoons or a chick flick doesn't hurt :)
I watch comedies...watch this episode of Whose line is it anyway. Funniest episode ever and after watching it believe me you will have no time to be freaked out about anything...well except maybe RIchard Simmons..HAVE TO watch this xD
I tell myself it's all fake. Fake scenes. Didn't really happen. Couldn't possibly happen to me.
The first time I watched a horror movie, I couldn't go to the bathroom without having my mom standing behind the door (in case something happens haha) . And I couldn't see into the bathroom mirrors without feeling shaky and concerned. And as soon as the lights would go off, I'd run like crazy to my bed and hide.
LOL hope my random experience made you smile.
Watch it again.
tell yourself it's not real. You'll be ok. It was only a movie.