Ok. While you're doing the stuff that results in you feeling this way - ie "eating too much shit", do you enjoy it? And is it worth feeling this crap later on?
YWBU to do nothing about this when you're so unhappy with yourself you can't enjoy the treat of a surprise night out.
First, you need to stop talking to yourself in this negative way and making yourself feel worse. Buy some decent clothes that fit you, too - even if it's just one or two cheap dresses on Asos.
And when you're ready, start eating healthily. Not a diet, a permanent change to your lifestyle where you and your family eat good, healthy food and do some exercise every day. I know that sounds twee as hell, but diets don't work as they invariably "end" and you go back to the old behaviours.
Instead, start when you're ready on eating well every day. No fast food, no biscuits, no crisps, no "diet" food, no fizzy drinks, just proper stuff like fruit, veg, meat, fish and full fat dairy.
Act "as if." Act "as if" you're a slim, healthy and happy person and you'll become that way in time. And be kind to yourself!