I'm yet another who is afraid of the dark...I used to lie in bed, completely rigid, afraid that if I closed my eyes and slept, something terrible will happen to my entire family. Sometimes I would fall asleep anyways, out of exhaustion, while other times I would dash into my parent's room and sleep with them.
I'm still afraid of the dark, but the fear has lessened with time. I'm not sure why. I guess that as time passed, I gradually realised that nothing is going to attack me the moment I closed my eyes, or that no catastrophe is going to befall my family just because I decided to sleep.
I think people are unintentionally dismissive because:
a) Unless they experience similar fears, they don't understand;
b) They may believe that by acting dismissive of your fears, you will eventually realise that your fear is unfounded. By appearing nonchalant, they might be trying to give you the impression that there really isn't anything to fear about the dark.
Perhaps you can try slowly desensitising yourself to the dark. Maybe you can try sleeping in your own room, with a nightlight [a light with an adjustable dimmer sounds like a great idea] and a stereo playing...then with the light off, and finally with the stereo off.
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Said the apple to the orange,
"Oh, I wanted you to come
Close to me and
Kiss me to the core."
Think you're ASD? Get thee to a professional!