Be frightened/nervous/scared of your own shadow
Definition 1 of 1
Pro Tip 1/3Overstated Fear
The phrase exaggerates someone's fear to show they are scared of even silly things. 
You jump at every noise; you're scared of your own shadow!
Pro Tip 2/3Not Literal
It is a metaphorical expression, not meaning fear of an actual shadow. 
Even though it was just a movie, she's now so nervous of her own shadow.
Pro Tip 3/3Emotional State
It implies a temporary or situational state of fear, not a permanent trait. 
After hearing that story, he's currently frightened of his own shadow.