I think many of us experience times like these. Times, especially soon after diagnosis, when we feel beaten, anxious or totally frustrated by it all. A complete change from a diet which holds no bars to one that can at times feel quite restricted is no mean feat. What you say about feeling quite well one day but feeling angry or blue the next day happens to non Diabetics too. Food is so wrapped up in who we are, it is sociable we celebrate and commiserate with it. We show love with it and derive comfort from it. Some people can't understand how difficult it is to change the habits of a lifetime.
So why do we do it? We do it because we have an eye to the future. Why are our feelings thrown by changes in bg on top of struggling with this huge change in dietary habits? That, to me is obvious, we are human and we need time.
I hope you feel more positive very soon, better days will prevail eventually.