I recently was forced to put my cat down because her diabetes got too bad. I first learned of her diabetes when she was 5 and had it pretty under control for 9 years. She found it well, but with her again her body could not maintain it anymore and she became very weak and a failing liver. My vet told me that only option would be to put her down, and even though I tried to push for some kind of medicine or surgery, but the vet said nothing would help, so I put her down. If correctly treated and maintained, the cat can live a long life with diabetes.
I've got a 17.5 year old cat who has lived with diabetes for the past twelve years. He's near the end - in too much pain from osteoarthritis and neuropathy - but with twice daily insulin (it took several tries to get him on a kind that worked) and diet changes, he's been my happiest, most long-lived cat to date.
I watched him being born in April 2000, so we have a special attachment...not sure what I'll do without him. But this goes to show that with loving care (and, alas, a lot of money and time) a diabetic cat can thrive and be happy.
Please don't put your cat down or worse, leave the diabetes untreated (a slow, painful death). If you can't afford or commit to treatment, you might be able to find someone who is willing to care for a diabetic kitty. Best of luck.
My cat is 15 and has had diabetes for the last 10 years. I give him insulin twice daily when he eats and I ensure he does not eat too much. He had special low carb food for the first 5 years but i switched to regular canned food with no issues. This has kept him healthy for the most part. I think in 10 years he has had 3 episodes of Ketoacidosis where he had to have IV and stay at the vet. These were scary times and cost $$$ but all worth it. Miraculously he recovered each time! Cats with diabetes need extra care and love but can definitely live a long and happy life. Please if you cannot afford or commit to caring for this type of cat try to find someone who will.