My pre-op diet was Protein shakes for breakfast and lunch, then a sensible meal for dinner. No sugars, low fat.
By day 3 or 4, I was big time into carb withdrawal. I felt just like you describe. I was sluggish, foggy brained, grumpy, and had a headache that would not quit. It only lasted a couple days, then I felt much better.
Hang in there. After surgery, it doesn't feel like such a hardship. In fact, the opposite was true for me. I had little or no appetite and had to stick to a feeding and fluid schedule.
PS: I lost less than 5 pounds in the pre-op diet. Some people really lose, but I sure didn't.