Feeling tired after eating usually comes from high glucose levels when you consume food stuffs that have large volumes of sugar and carbohydrates in them such as bakery or sweet food items.
"It doesn't matter what I eat or how much"
Question is, are you eating a diabetic diet/nutritional meals? Do you feel the same when eating lighter foods such as green salads, fish, chicken and nuts and vegetables?
You need to help us help you by providing information on how you control and manage your diabetes; when you test, what your preprandial and postprandial test results are [include last A1c], your body weight, whether you exercise daily or not, and types of diabetic medication/s.
Have you measured your blood sugar before a meal and then after the meal (usually measure at 1 1/2 - 2 hours after a meal). if your sugar is going high or low after eating this can affect how you feel.