Your reference to getting dark might have a parallel with pilots to fly too high and do not get enough oxygen. They get what is called "tunnel vision" This is where the outer cells of the retina lose the ability to respond to light first.
If this is similar to your experience, you may wish to get more "light" (or oxygen) in your life.This might be relational connections, interests and activities, or experimenting with activities you might never have considered.
Do not go gentle into that good night.
Rage, rage against the dying of the light.