Welcome Toni! Don't worry--with the right treatment, diabetes is very treatable in cats. Bandit has been diabetic for 7 years now, and he's still doing great! He was also diagnosed at age 7.
The best way to know if his insulin is working is by home testing his blood glucose. Did your vet recommend this? Unfortunately, some vets do not for whatever reasons, but it is absolutely vital to getting his diabetes under control. You can use any home glucose meter to do this--one of the popular ones around here is the Relion at Walmart because it's inexpensive and easy to find strips.
The only prescription (vet) diet that is appropriate for a diabetic cat is Purina DM canned. Anything else you vet gave you is going to be too high in carbs for a diabetic cat. However, you don't need to feed a prescription food--there's nothing special about them that makes them better than many commerical foods for cats, so all you're paying for is the label (Purina DM is virtually the same as the Fancy Feast Pates, if you look at the ingredients). You just need any low carb, canned food that is less than 10% carbs. If you visit
http://catinfo.org/, there's a
chart that lists many commercial foods and their nutritional breakdowns--just pick any food that is less than 10% carbs.
What insulin did your vet start you on and what dose?
I know this all seems overwhelming at first, because there's a lot of information to absorb, but if you have any questions at all, please come over and post them in the health forum!