Older cats very often have urinary problems. I used to think my old cat was “forgetting” as you said. Now I have learned that when a cat uses the litterbox and it HURTS, as it would for a bladder infection, urinary obstruction, etc., the cat associates the litterbox with the hurting, so avoids it. Makes sense, no?
MOST older cats get kidney failure. They drink a lot and pee a lot…. it means the kidneys are only partly functioning, not doing their full job. The cat gets dehydrated, because the kidneys aren't returning enough fluid back to the tissues. Subcutaneous fluids — which sound difficult and scary — are really lifesavers for these cats. And really, REALLY, it's not that hard and not that big of a deal. I've been doing subQ every day since last November, and my 16-year-old Jake is very used to it now. His kidneys don't work well, but the subQ fluids really help him feel better. To me, if he is purring every day, eats well, and can sleep hard and happily, that means he is feeling pretty well — so far!