After reading 20 pages, eyes blurry, can't read anymore, what could be wrong?

Blur: The most common cause of this type of symptom is dry eye. Associated with this is a probable decrease in the blink reflex which can occur with prolonged near vision,reading,computer use etc. Try using lubricating drops. If no help see an ophthalmologist.

Presbyopia: At 41 you are probably getting presbyopic, your eyes are losing their power to focus. As we age, the lens hardens, the ciliary muscle weakens and it is harder to see up close. The fact that after prolonged reading, you are having problems makes this seem likely possibility. It won’t damage your eyes to read. When Social distancing guidelines allow, get an eye exam. Could mean reading glasses.