Don't worry about it too much. This is not specifically a spectrum problem. Some people never fully outgrow a fear of the dark, others do, but at different rates, so you may just take longer than some of the others.
I was terribly afraid of the dark as a child, but gradually outgrew it. I finally got to where I would occasionally walk down to the store in town, in the dark, although there where street lights. The shopping center was also lit up, but away from the center, it was still somewhat dark, even with the street lights. With one dog we owned we never had a fenced yard at any of the places we lived while owning him, so I had to do some of his walks in the dark. He and I were okay with that. We sometimes saw cats, opossums, mice, once a family of raccoons peeking out of a sewer grate, once a deer up close, and, unfortunately, twice he went after skunks. I have a weird sense of smell. I can smell some things, but not others, including skunk. I was the only one in the family who could stand to be near the dog even after he was washed with tomato juice.
I now kind of like the dark. Because of several health problems I have, my schedule is very irregular. I am often up or asleep at odd hours. If I fall asleep in daylight, and it's dark when I wake up, I like just staying there for a while, enjoying the peacefulness of the dark, before I have to get up and turn on a light, so I don't trip and fall on the way to the bathroom. If it is dark when I come home from running errands, I like to stand outside for a couple of minutes looking at all the millions of beautiful stars overhead. You can only see them in the dark.
If you are worried about bogey men, put up a few posters of Chuck Norris and other nice tough guys, and just think of how peaceful it is in the dark, and of all the neat animals out there in the dark.
Remember, the bogey men and the monsters under the bed and in the closet are afraid of Chuck Norris. Remember also, that we on the spectrum are all:
A Different Drummer
If a man does not keep pace with his companions,
Perhaps it is because he hears a different drummer.
Let him step to the music which he hears,
However measured or far away.
--Henry David Thoreau