I've struggled sometimes too. Try increasing your font size. My vision is excellent, but I think staring at tiny fonts increases strain on your eye muscles or something.
Also, as monitors have gone up in resolution over the years, pixel density has increased, shrinking the default font size from what is appropriate for the eyes.
Lowering the brightness is not always beneficial. In fact, the opposite can be true. Brightness is usually controlled electronically by rapidly switching the backlight on and off. Therefore, lowering the brightness can increase the amount of flicker, which is imperceptible but still implicated in strain. Try turning the brightness all the way up, which will minimize flicker. If it's too bright, wear sunglasses and see if that helps.