My cat Y.Lee was diagnosed with diabetes. It was no problem giving his twice daily injections. His new diet, which was a high fiber/low fat dry food was not much more expensive than his old food and he liked it. What he did not like was all the blood testing that needed to go on in order to try to regulate his blood glucose levels. We did all the right things for 6 months and he got somewhat better but he never became completely regulated. When he stopped eating for 3 days we decided that no more testing be done (for this made him most irratable) and put him to sleep. BUT I would not have put him down without at least trying !!!!
Cats are the only animal that CAN BEAT diabetes. If the cat will tolerate the treatments, if the extra care (vet visits, insulin, needles, special food etc.) will not be too great of a financial burden then your kitty deserves every chance that you can give him.