Aw, big hugs, Thistle! Sounds like you've had a rough go of it for the past while.
A couple of things: first, the gestational diabetes thing, while annoying and time-consuming to manage (you have my sympathy!), is not a crisis -- your baby will be fine (and so will you). I know three women who had it. None of them was diagnosed until halfway through her first pregnancy, and even so, none of the babies was enormous. All were born vaginally. Not totally sure why you need a planned c-section? But I agree with Joy; an appointment with a dietician is important, if you haven't had one already.
Secondly, don't worry too much about the c-section. If you do have one, it will be fine. I know so many women who had one, and they all managed. I've had a similar surgery and the first few days were rough (but you will be in hospital, with help from nurses) and after a week you will be moving around fine. It takes six or so weeks before normal pants are possible, but that's a drop in the bucket of your life! Also, even though I had run-of-the-mill vaginal births, I was nevertheless wearing yoga pants exclusively for at least six weeks postpartum!