Just a long shot, but go to your doctor and have a blood test for prolactin. I had the same change in my vision, where my night vision got worse very slowly over a few years. Turned out I had a brain tumor. The prolactin test can rule out the most common type of these tumors that affect your night vision.
Chances are 1 in 100,000, so it is not likely, but if it has been accompanied by weight gain and growing fatigue its more likely.