Are you feeding as well as watering? Have you repotted at all in the last 3 years? If not it could well be pot bound by now.

Yes, I agree with Moon grower, it's likely pot bound. Certainly repotting and feeding can't hurt!

Only two other unlikely ideas occur to me, that someone else in your household could have started watering it as well, so too much water. Or a cat is peeing in it. I lost a plant under mysterious circumstances and was later able to figure out my new kitten thought it was a litter box. But not in time to save the plant.

Good luck!

You have to love cats SM

Indeed! Lucky for her, she was impossibly, ridiculously cute and so suffered zero consequences for her actions. The solution to that problem, I've since discovered, is to put decorative rocks covering the soil.