Did you vet want her to gain weight? I know for humans if their diabetes is really bad for a long time they will lose weight, but overweight is more common in the development and makes managing the symptoms even more difficult. Once you get her insulin under control, I think her weight will come back on even if she doesn't overeat and some think being a little underweight is better.
Regarding diet, you might want to become familiar with the glycemic index that ranks carbohydrates based on their rate of glycemic response (i.e. their conversion to glucose within the human body. Read More
For example, choosing green beans over carrots might be a good idea. Glycemic Index – NutritionData.com Also, eating too much at once time throws blood sugar off, so smaller more frequent mills might be advisable. Has the vet said anything about the kidneys? If diabetes has been untreated for a long time, it can damage the kidneys, that information is important when selecting a food too.
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