The first monster you have to scare the audience with is yourself. - Wes Craven
When did you last scare yourself? When were you last really scared by something and what was it?
Fictional monsters like zombies, werewolves, vampires and such are not nearly as scary as real things like mortgages, collection calls, being unemployed, law suits, health problems, danger to children and other family. Real things are far scarier because there is less we can do about them and they are real, part of our reality. It is so much easier to read about fictional monsters and just close the book, finish watching the video, at the end. They all go away but for the memory. They go back to being unreal.
Think of the scariest thing you have faced and write about the feelings but change it around to one of the unreal monsters. Put all the fear, the real fear, into an unreal monster. See how real you can make it feel.