First, you should change to a veterinarian with an x-ray machine who puts more effort in helping your cat. How did he arrive at the diagnosis of diverticulitis?
X-rays will show how much feces is actually in the colon.
Has your cat had any kind of diagnostics, like bloodwork? Afterall, at 8 years old, she's a senior cat.
Conditions like kidney disease, diabetes and hyperthyroidism can cause constipation.
Do not use any kind of enema you can get in drugstores, do not administer enemas yourself, the vet should do it.
Things that can help constipation are:
Fiber (there are veterinary diets with added fiber), as much water intake as possible (use canned food instead of dry), pumpkin, certain laxatives (lactulose, miralax), probiotics.
If it is a severe case of constipation, sometimes hospitalization on IV fluids is indicated.

https://www.petmd.com/cat/care/8-way...onstipated-cat

I hope this is helpful.