Just wanted to offer my support too. I too was diagnosed with GDM recently (my lucozade test thing was fast over night, then predrink level, and 1hr and 2hr post drink levels - My fasting and 1hr were a little elevated but 2hr was OK). My ultrasound has revealed my LO measuring about 1.5w large in general, Biparetial Diameter - 3w large!, and my bump is measuring 32w, whereas I'm only 28w in reality!!
I've been doing home monitoring blood glucose with a finger prick for the past 2weeks now, and thankfully have been getting OK numbers out.
Like you mentioned, I've been aiming at a Low GI type diet and it's working well. You'd be surprised how quick the sugary cravings dissipate and you begin to not even miss the sweet stuff.
My diet has comprised of wholegrain and complex carbs (brown basmati, bulger wheat, Burgen bread, whole-wheat wraps and pittas etc) lots of lean meat (chicken, turkey mince etc), fish (mackerel, salmon etc), some full fat dairy (greek yoghurt, cottage cheese etc - I'm keen not to low fat and low carb - some fat in moderation is good and help slows the whole carb digestion again), eggs in a multitude of guises, and greens (spinach, broccoli, pea shoots etc).
The trick I found is to get creative with a bunch of ingredients you find work well for you and your blood sugars. I even had pizza-y toast things this lunch time as I had a pizza hankering - Burgen toast, tom puree, oregano and black pepper, chilli & lime chicken pieces, and a smattering of extra mature cheddar cheese. Tasted amazing and didn't even come close to spiking me over my threshold values!
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Sending you hugs, and don't feel guilty at all (I was a bit guilty and teary about mine last week, especially as my diet hasn't been great and I am overweight, but have since met loads of women who've said "yeah I had that", and they've been stick thin and virtuous with their diets - so it can get anyone, anywhen - just pot luck. In fact I even found an article suggesting a perfectly good biological reason for women to become insulin resistant in the 3rd tri as it ensures the foetus has an adequate supply of glucose and it's not taken up by the mother, it's just some of us overshoot/go a bit further and it turns into GDM). Anyhoo, hugs to you.