Welcome to FDMB, although I'm sorry you had to find this site. My name is Shelly, and my cat's name is Jersey. We are one of the many success stories of FDMB. Jersey was on insulin for just a few months and then went into remission. She is now diet-controlled.
You've been getting some great advice, so I'll just add in my personal experiences in case it helps. Jersey was diagnosed in November and the vet recommended Hill's w/d dry food first. He said that some cats can go into remission from diet change alone. In January of this year, her numbers still hadn't come down, so he prescribed Humulin. That's when I found this site. I learned that the w/d was high in carbs - it was just as bad, in fact, as the Meow Mix I had been feeding her. I learned that diabetic cats really do best on a low-carb diet (less than 10% carbs). I also learned that Humulin is not the best insulin for cats because it brings the numbers down quickly and then wears off just as fast. Lantus, Levemir, & Prozinc are the best insulins for cats. I also learned that home testing is a crucial key in getting diabetes regulated.
We made a food switch to low-carb food and went to a different vet for a Lantus prescription. We also started home testing. It took a while (about 6 weeks) on the Lantus before we started to see Jersey's numbers come down, and it wasn't long after that before she went into complete remission. Some cats become regulated very quickly; some cats take a bit longer. And some cats - like Jersey - go into remission.
On the food - I agree with what others have said. It's really important to make sure you're testing before you make the food switch since the insulin dose may need to be lowered when you do that.
On the home testing - Most people here use the ReliOn brand meters from Wal-Mart because they are a lot cheaper. (Some vets recommend pet-specific meters, but the test strips can run about $1 each, so that gets expensive quickly.) We used the ReliOn micro meter. You'll need to get the meter, the matching test strips, and the lancets. We also recommend Neosporin ointment with pain relief. The ointment can be used prior to poking to help the blood bead up and after poking to help the site heal. I know home testing sounds scary/awful, but it really isn't as bad as it sounds. And people have tons of tips to help you learn how to test successfully.
On the insulin - Which insulin are you currently using - Lantus? We base the dose on how low the insulin takes the kitty - that's what we call the "nadir." For Lantus, most cats usually reach nadir around 5 to 7 hours after the shot. Getting in some tests around that time can be really helpful in figuring out how well the dose is working. I know the vet has run some curves, but kitties often are stressed out at the vet's office, which naturally raises their glucose levels. Testing at home, without all of that vet stress, can give you a better idea of what's really happening with your kitty's numbers.
My cat urinated outside of the litter box, too, so I know how frustrating that can be. The good news is that once the numbers come down, the accidents will stop.
Hang in there - you're in a great place to get help. If you'll stick with us, I really believe we'll be able to help you and your kitty. (What are your names, by the way? I hate just calling you "you and kitty." :lol
Shelly