sweetie.....huge hugs it must be a big shock for you. just know you are not to blame at all. GD is not related to diet, exercise or lack thereof in most cases. You have just been plain unlucky tis all. scuse the poor typing have just got a new puppy and am typing one handed!
in simple terms here is how the pancreas works during pregnancy: during pregnancy you will require 2 -3 times the normal amounts of insulin to metabolise your food, that goes for everyone when they are pregnant. Understandably this places stress on the pancreas to produce that much extra insulin. In some people for whatever reason the pancreas doesn't cope as well with the extra stress and can't quite keep up with the extra demand so you get a small shortfall of insulin. you have just been very unlucky to be one of those people.....not that it makes it any easier but it is not your fault.
there is a fantastic book i reccommend you look for called the new glucose revolution by professor jennie brand-miller. it explains the GI approach to diet extemely well and has tips and recipes etc. Also the diabetic living magazine comes out every 2 months and is awsome for food tips for diabetes of all types.
chin up keep thinking its only 12 weeks tops not a lifetime.