Yea, gestational diabetes sucks, and it is important to keep under control, otherwise you could end up with diabetes after the baby is born (as in, it won't go away). But at the same time, you need carbohydrates (sugars) to function, hence the low blood sugar/shakes etc.
I would suggest that you go back to your doctor with those symptoms and explain how you are feeling, because low blood sugar is not good either.
You could also be low on iron, (anaemia) can cause the same symptoms. I had some problems with this when I was pregnant and after I had our son. Try eating foods with more iron, spinach, red meat...but do not drink milk with these foods, because the calcium interferes with the uptake of iron.
Be sure you eat when you are hungry, that is important. Your body knows when you need to eat, if it tells you to eat, listen. Try and have some low carb snacks on you, even granola bars, crackers etc so that if you need to eat you can have something.
Were your sugar levels really high for the gestational diabetes test? Did you have large amounts of fruit or juice before you did the test? Sometimes it can result in a false negative.
Defiantly go back to your doctor though. It is important to get this under control.
Best of luck, I hope this helps!