hi op. firstly you sound very down on yourself, and if you want to dress better you need to start feeling better about yourself and focus on the positives and accentuate them. maybe try to watch some episodes of gok wan or trinny and suzannah to learn how to look at yourself realistically and not focus on negative things. they also have books that may help, you could get them from the library if you don't want to buy them. they will help you learn how to choose clothes in fits and colours that will flatter you.
I always find if I feel like I've gotten stuck in a rut the first thing I will look at is my general grooming, maybe get a haircut/colour, look at your skin and if needed get a good cleanser & moisturizer, paint your nails, sometimes I get my teeth cleaned and polished as well. once thats done, maybe invest in some new underwear and go and get fitted for a new bra or two.
wardrobe wise you could maybe look at investing in a few staple items of clothing, for example if you like tracksuit bottoms, maybe switch into linen trousers and some new tees or vest top and cardi combos for the summer. they always look smart but are as comfortable as pajamas! you cant really go wrong with a pair of bootcut jeans if you carry weight on your legs, and you could couple the jeans with some simple tshirts and a pair of low heeled ankle boots for day to day wear. add a basic fitted blazer/suit jacket and you've got a smart comfy out fit. to top it off you could invest in a few accessories like an inexpensive bangle, earrings/necklace.
I hope this helps, I know what its like to get stuck in a rut like that and its easy to feel like you just don't know where to start.