Hi there.
I'm 32 and slightly bigger than you (14), btw, but probably quite similar in terms of wanting to look vaguely fashionable, but never having been uber cool .
I mix and match from lots of different shops. There is no one shop I love, and accepting that has been good for my shopping experience .
I have also found a style that suits me (I think/hope!?), which is 'day dresses' that have a drapey front to mask up mum tum, are a bit fitted around the waist to accentuate that and then a V neck to accentuate decent bust. I wear them with black leggings/thick tights and boot in cooler weather / leggings or bare legs & sandals/flip flops in summer. It's got me out of a jeans-and-tops rut, and I can dress the style up for more formal occasions, too. I'm presuming you don;t have to dress ultra formally for work?
Shops I find stuff in:
White Stuff - always shop online, as none near me, and usually try to stick to their sale stuff.
Great Plains - ditto
Fat Face - ditto
Top shop for a few basics - but I also tend to shop online, as the stores give me a headache (too many 14 yr olds giggling )
M&S for a few basics - I quite like their slim leg dark blue jeans and their extra long black leggings for underneath dresses/tunics.
Zara - lots of it is too flimsy and young for me, but I do find one or two nice things most seasons. They are good for jackets and 'on trend' shoes, I find.
Dotty P - has got a lot better in recent years. Also nice and cheap. I buy a few bits here in quite regularly.
Warehouse - a very young shop, but does have the occasional lovely piece for quite cheap. I got a great print dress there a few months ago that is very versatile and was only £30.
I must admit, I do like Primark and New Look, but I think they are only worth a visit when you already have some decent basics and are looking for accessories - jewellery, scarves, cardis etc - to add to an outfit, or for summery stuff for very hot weather (not worth spending much on hot weather clothes for this country!).
Hope that helps a little bit. I actually hate clothes shopping and find it stressful, so have got it down to a fine art in terms of which shops I know I might have a chance of finding something in and which are just a complete no go.