Oh you poor thing. The gestational diabetes thing had me in a real tizzy when I first got it and it was not my first baby either, which in itself is new and daunting. Make sure you see the community midwife if she is nice, as well as the clinic. I find the community midwives make me feel as if I'm going to have a baby and explain everything about that, but the clinic makes me feel like a medical problem!
I was on insulin during my last pregnancy, and the clinic told me the baby was big one week, and small the next and had me in constant confusion as to what blood sugar levels were OK. In the end I had a tiny baby (just 6lb 6 compared to the previous ones which were over 8lb) and he was fine. The diabetes is such a nuisence, but when you hold your baby in your arms for the first time and gaze into its eyes, it will all be worth it - every last second of stress and worry, not to mention all those needles.
It is normal to have some fluctuation in blood sugar results - there are just too many things which affect it to take everything into consideration - exercise, stress, food, illness to mention just a few.
Ultrasounds are not always accurate, so try not to let them worry you too much. Babies have growth spurts before they are born just like they do afterwards, and do not follow the nice even lines of the charts. It is only if baby is consistently getting much bigger than average that you need to worry.
If you keep a good food diary, that helps. 2 units per meal isn't a lot - that used to lower my blood sugar about 1mmol at most.
Carbs are not good, no matter what the dietician/clinic say - salad, omlette, fried breakfast, meat and veg is good. I often have loads of cauliflower instead of pasta or potatoes. It is so difficult, as you have try to achieve instant control, and you don't have the time to experiment a bit in a relaxed way.
The book which has helped me most is a tiny book, a "Collins Gem" carb counter, which details the amount of carbohydrates in food. If you look on ebay for "carb counter" it is only a couple of quid or so, or it is available from amazon etc.