Hi - my cat has also been recently diagnosed as diabetic.
She was overweight, but had gotten thin due to the diabetes. I sure understand being so concerned about getting the weight back on.
I've learnt that when the diabetes starts to get under control, the kitty will start to gain weight. Slowly, using glucose curves at the vet, her insulin level was increased. She started at 1 unit and is now at 4 units twice a day now.
I started to notice a difference after 1 week. It was definitely an improvement. After 3 weeks and an increase in her insulin to 2 units, her weight loss stopped.
Next, she started developing muscle mass. In 4 weeks, she had gained weight and became stronger. It's been about 2.5 months and she's back to feeling solid. She's calmer and more interested and playful again, eating less than she did at the start.
Basically, I fed her the high protein, low carb (<7%) diet as much as she wanted and kept it fresh. I bought Friskies Special Diet Turkey and Chicken which she devoured. Suddenly, she wanted something else, so she's on Fancy Feast - I just posted the list of foods and added an attachment under help with diet thread. She's been eating Fancy Feast Classic Chicken Feast, Fancy Feast Chunky Chicken Feast and Fancy Feast Chunky Turkey Feast. After about a month, EVO hard food was added. She doesn't eat now since it was recalled. When it becomes available, I'll use it again.
Hope this helps.
Initially, I tried the more expensive brands, but she seemed to prefer the lower fat of the Fancy Feast and Friskies. EVO has a high fat content, but that was acceptable to her. I just bought multiple cases of food because she would eat like crazy and I didn't want to run out.
Her glucose is still too high. However, because she's gained weight and is stronger, the vet will do a teeth cleaning and likely some extractions. The infection in her mouth may be contributing to the requirement for more insulin. So, even without her glucose completely normal, she has gained all her weight back and developed her muscle. Happy kitty.