Had one. Took a month and a half to get the insulin to food ratio correct. Or, perhaps, it took her that long to get used to it. She lived another 16 months. We never gave her any "treats". Only the measured food for diabetic cats... and lots of attention.

Do not dispair. It will work out. Get yourself a scale and weigh the cat twice a week. Keep your own records regarding insulin, food, weight, blood sugar levels, siezures, and activity. Above all, give her the insulin within 15 minutes either way of the same time every day. I realize that may be inconvenient, but not for the cat.

Good luck and a prayer on you and the cat.