M.R.
I failed the 1-hour and the 3-hour...twice! Imagine that torture.
Remember that it could just be your hormones messing with the release/delivery of insulin and there might not be anything you can do about it. I know that when I failed my first 3-hour I immediately went on a diabetic-style diet. 4-6 weeks later (can't remember exactly when) I retook the 3-hour and got the exact same numbers. On the bright side, I delivered vaginally at 37w1d and my son was 7 pounds.
Don't worry about the ultrasound - if you are having issues with GD your doctor could be concerned about the size of the baby and may want to keep an eye on things. Unrelated, I had a cerclage at 13 weeks and had to go to the doctor every week and get an ulstrasound, and it was no big deal (my kid is now 2+ and fine).
A change in diet is always helpful; diabetic diets are actually very healthy for most people. Exercise will also help your body better manage labor and delivery. At the same time do not beat yourself up if the GD does progress, the baby does get bigger, and for the HEALTH and SAFETY of you and your baby you have to have a c-section - you probably won't know until you get very close to the due date as to whether this will be a possibility. Besides, no one gets a prize for 'doing it better' and delivering a certain way!
Try to stay calm and positive and don't blame yourself. It is highly likely that it is just the 'pregnancy' that brought this on, nothing that you did or didn't do. Good luck!