I'm scared of mirrors in the dark
Why do they bother you?
They're full of the night's terrors
Mirrors are just reflections of the real world, so the shadows that you see in there are nothing that can hurt you. You should face your phobias as an adult, but if you can't, you can always take the mirrors down, or keep lights on at home.
Don't you sometimes get scared of your reflection in the dark?
There have been times that I have definitely been startled by a flash of light or a moving shadow in a dark room.  However, I recognize that this is just my brain playing tricks on me, and I hope you can do the same.  There is nothing sinister about a mirror, and all you are seeing is shadow and light.