For some reason, I think there's something more upsetting about getting sick when you are being careful, you know? If you were not taking as much care and you get sick, it's more like 'dang it, I knew i shouldn't have done that.' But when you are taking care - and often missing out on foods and activities you may like as a result - it can be so disheartening.
We've all been there. I was in the exact same place a couple weeks back, reacting to things that I'd been trying to avoid and feeling SO down about it. But...the mood goes back up, yeah? Maybe you could even put a ridiculous little post it note put up in a few places to remind yourself that it will.
Or listen to some really uplifting music.
https://www.youtube....h?v=L0MK7qz13bU (Like this one - it's even the sing-a-long version, heh. ^_~)
One thing that might help is Pinterest and paleo recipes. These recipes will be grain free, dairy free, and legume free. Some have nuts, but many more involve meat. They are very often processed-food free. Pinterest has some amazing paleo recipes on there now - it's like a giant cookbook geared toward people who have to cook with whole foods.
Makes it easier than trying to figure out all these recipes yourself. See if you can find some easy-to-make ones with some of the foods you have on hand right now, even. Might make you feel just a little better. Even doing something like chopping a banana into pieces and putting it in the freezer, or freezing some blueberries, would make you a quick, cool snack food you could have right now for little to no effort. It just seems like that might be helpful right now, yeah?
Take care - you can do this, and other folks going through it are always willing to listen if you need to vent a bit.