I had a diabetic cat for many years. Glucotrol pills didn't work, so we had to do insulin injections twice a day. Other than a slight crimp in our lifestyle to be at home twice a day (or have someone else there) to do the shots, the whole thing was quite manageable. Watch how much water the cat drank, how much she ate, make sure urination was normal, the occasional urine dip, month blood tests at the vet, twice daily insulin shots. The cat did not even notice the injections, which are VERY easy to give. The cat had a good long life after the diagnosis.

Good luck with your cat.