All tests show that my vision is perfect, but to me everything looks much darker, and less vivid than it used to be long ago. what is this? brain fog?

Check it out: Any visual disturbances should be evaluated by a eye doctor to determine if there is any medical cause. A simple exam to determine your visual acuity may not be adequate enough. Consider seeing an opthamologist, who is a medical doctor with specialized training diseases of the eye. Also consider seeing a neurologist.

Did you get: A flourescein angiogram, to see how the blood flow is to the retina/choroid? Maybe an oct may be helpful.