I had a diabetic cat. Diagnosed around 12 years old and passed away at 18. He didn't even care about his insulin shots. He would be just happily munching away his breakfast or supper and not even notice he got jabbed. He was always his super friendly and cheerful self until his time came. As long as you can maintain their medication they will be just fine.

EDIT: There are also specialized foods for diabetic cats, some available directly from the vet. If they recommend feeding your fuzzy one that, it's worth it. Way better for them than the stuff you'll find in most pet stores. At least that's the situation in Canada.