Hi Andi! Welcome.
Unfortunately, very few vets get all the aspects of successfully treating diabetes in cats right. Vets are general practitioners--they have to know a lot about many different medical issues in many different animals, and typically have incomplete or out of date information when it comes to feline diabetes. Some vets are also hesitant to recommend certain important aspects of treatment because they think their clients will be unwilling to comply with those recommendations. Things like daily home testing, all canned diets, and good insulins like Lantus (because they're so expensive in the US), sometimes scare newly diagnosed owners, but none of these are as bad as they seem with a little support. I've attached an article to this message if you need information to print and bring back to your vet to get them on board with the treatment guidelines we recommend here. The
AAHA guidelines for Diabetes are also available online to bring to your vet.
Your vet is definitely not dosing the Lantus correctly. Dose adjustments need to be made in small, .25-.5u increments, and Lantus should always be the same size dose in the AM as the PM. I agree that the dose was raised too much, too quickly. This can keep BG uncontrolled and high just as much as not enough insulin. Cats rarely show signs of hypoglycemia unless it's gotten to the point over time of being life threatening--their livers dump glucose into their bloodstream to counteract the low blood sugar, so low numbers are typically followed by several days of very high numbers. That's why daily testing is so important in determining the right dose--office curves are typically inflated from stress and these rebounds from low blood sugar, leading to chronic overdosing. They don't give you a good picture of how the insulin is actually working in your cat--and it's much cheaper to home test. You don't have to pay for office curves or any other diabetic testing at the vet if you're home testing.
Chris gave you some great information up above--there are three things that are key to getting your cat's diabetes under control: A low carb, canned diet (there's a TON of commercial options--check out that link to the canned food list), dose adjustments via daily home testing (this is NOT as hard as it seems--even the most fractious cats accept it after a week or two of routine and diabetic safe treats), and slow acting insulin like Lantus (which is WAY cheaper if you buy from Canada). The large majority of cats will go into remission when these three things are done.
One important note--many diabetic cats need drastic decreases in their insulin dose or even go into remission completely when you switch the diet from dry to low carb, canned. It's very important to make sure you're home testing and lowering the dose when you change the diet--otherwise your cat could have a dangerous hypoglycemic incident.
This seems overwhelming at first, but once you get on a good treatment path and routine, things really get easier! Bandit's been diabetic for 8 years now, and I actually think he's been
healthier in the long run because of it because it made me learn about feline nutrition and make much healthier decisions for him.