My heart goes out to you. My precious Mordred had diabetes for over two years before she added kidney failure to the mix.
My vet prescribed insulin for Mordred; and, I gave her two shots a day. The vet will show you the patch on the back of your cat's neck where you give them the needle. They feel very little pain here and I got to the point eventually where after I had loaded the needle, I could give her the shot in 5 seconds or less.
I used to put her on the counter away from the other cats and gave her the food all to herself. While she was eating, I gave her the shot and then left her there to finish her meal.
Go to the vet, there are other foods your cat can eat and I'm sure he/she will find one she/he likes. Check out the insulin situation. I'm sure your cat will need it and it's relatively inexpensive.