That was certainly my reaction once I'd grasped I was not only fat, but I'd given myself type 2 diabetes (well, given there's a family history, probably moved it forward a good few years). You are right, there is simply no excuse. The advantage I have I guess is I'm pretty reclusive so it was actually pretty easy for me to change my eating / exercise pattern as no one else is effected and I'm not pressured to attend fancy restaurant meals / pub lunches etc.
I strongly advise learning the calorific values of what you eat - some food has loads, some next to none, so you really don't have to starve yourself at all. You can still eat nice big meals just by say swapping pasta for brown rice and say a cheese and mushroom sauce for chopped tomatoes. Just research everything. Also be wary of so called 'health food' like muesli etc, a lot of it is surprisingly high calorie, and you'd not expect it to be. I try to never eat something without having a pretty good idea of the calorie content. It's all just a basic input / output issue, so apply your brain to it.