You may know how to take care of cats, but you don't necessarily know how to take care of old cats.
If the doctor tells you to give medicine to the cat, do it exactly the way he says, even though the cat seems better and doesn't like the medicine. If you don't, the cat will eventually get sick, especially if it has to do with the intestines.
Muscles on the inside of old cats get frail and weak just like the muscles on the outside. The intestines are muscular and they get weak too. Eventually the cat will get too frail for difficult bowel movements. If the vet tells you to give the cat a laxative, don't stop it.
It's sometimes difficult to tell if the old cat is constipated. Ask your doctor how you can feel the cat's abdomen to tell if there are a lot of stools inside. If you do it gently, your cat will think it's getting a nice massage. It will also help to stimulate the intestines. If there are stools inside do something about it right away.
Old cats can die from constipation. If you don't follow the doctor's directions or keep good track of your cat's digestive health, she can get so badly constipated she won't have the energy or strength to do any more about it.
Ask your doctor of other problems you can expect, find websites, buy a book.
Old cats, especially if they're deaf, sometimes yowl, especially at night. I think they partly just want to hear
When in doubt, take your cat to the vet. The old cat is frail and needs help much sooner than a younger cat.
If you follow this advice, I hope you'll avoid nagging guilt for the rest of your life.