My first thought is that you're going a little too fast on the food introduction. When nausea comes after some time, and it coincides with diet changes/increases, my suspicion is that it's likely a reaction to that.
Back off some of the foods, take small bites, chew, chew, chew, and keep your meals small. You probably won't notice a difference immediately, but given some time for your stomach to calm down, it should help you. Don't worry so much about nutrition at this time. You're recovering from major stomach surgery, and it's expected that your nutrition will be down a little. Your first priority now is calming that stomach down and healing.
The other question is that are you getting diarrhea with the "diabetes symptoms"? Some people have dizziness and hypoglycemia-type symptoms with vagus nerve irritation and dumping syndrome. Just a thought.
Hang in there!