In my lifetime I have been everything from a size 6 to a size 18 (and I'm just 5 foot one and don't wear heels much - it hasn't always been easy to find things in petite sizes!) I've been every shape from pear to hourglass. Now I'm definitely middle aged and in between a size 14 and a 16, but even that varies from shop to shop - some think I'm a size 10 still and others think I'm up to size 20. So the advice about trying things on is very wise.
Maybe make a day of it, and go shopping on one of the quieter days, a Monday or a Tuesday, in the morning, when you're less likely to be surrounded by 6 foot tall one inch wide supermodels or teenagers! On a landmark birthday a few years ago a couple of friends and I went on a coffee morning/shopping spree and our rule was that we each had to try on three garments we wouldn't normally have touched with a barge pole. It was actually surprising to discover that some of them suited us!
And it even varies in the same shop sometimes as different batches often seem to be made on a different model/dummy (yes, M&S, I'm looking at you).
I think I'd be inclined to have a look at a capsule wardrobe idea - dark trousers are a great foundation, but how about brightening up the dark trousers with some nice floaty tops in bright colours, or a pretty scarf or a nice shawl around a dark top? Or, as others have said, some nice earrings or a signature necklace?
Have a look on ebay in the larger sizes. Even if you don't want to buy, it'll give you an idea for what sort of stores or labels to have a look at, although many of them have sadly gone under in recent years. But a few are Simply Be, Long Tall Sally, Very, Yours, Curvissa, Pink Clove, and Evans. Just having a quick look round the Internet, there are some celebrities out there that aren't tiny little pixies - thestir.cafemom.com/beauty_style/213301/plus-size-celebrity-fashion/240193/ava_duvernay/10 has some great ideas (though some are pretty over the top too).
Another option might be to have a go at making your own clothes? There are masses of great classes out there and it's really popular right now, plus you'd get to meet people with similar interests. It also means you can choose much more interesting fabrics than most of what's out there on the sales racks in high street stores.
I have a few friends who sew and knit and crochet, and I'm personally a huge fan of vintage and charity shops and it has always been my sadness that I can't sew much. I often see lovely things that are either too small for me (could be let out) or too big for me (could be taken in/shoulders adjusted). I can manage simple alterations, and I've done things like change the buttons on jackets to make them more up-to-date and personalised, but anything much beyond that is beyond me. I have learned a lot about fabrics and labels that way though - in terms of which ones are a good match for me, I couldn't care less about whether vacuous fashionistas think they're suitable or not. I've often been able to try fashions via charity shops that I couldn't have afforded from the high street. (We donate a lot too.)
I've also found the shop assistants and volunteers in most charity shops a lot less snotty than many of the ones in many high street fashion stores and concessions. Including John Lewis...and I'd never even contemplate sticking my nose round the door of a private boutique, especially if they're not displaying any prices on their garments in the window.
Also what about doing a Colour Me Beautiful session or similar? A lovely ex-colleague of mine once hosted a DIY version where we had white sheets that we wrapped ourselves in, toga style and up to our chin and then we held various colours up to our faces to see which shades were our best colours for our skin.
And I wasn't a huge fan of Trinny and Susanna (I much preferred Gok) but some of their advice was good. I disagreed with them on some of the ridiculous kitten heels, and their ludicrous bright colours. I'm sure they went down really well in Belgravia, but they really weren't appropriate for a small "c" conservative working environment in the rural West Country. I also didn't care if that bra shop in London was "by Royal Appointment" the one and only time I steeled myself to visit their bras weren't particularly suitable for my shape. I've had more luck with American shapes! And on the trousers thing, at one point I had around 7 pairs of the exact same M&S black trousers, having finally found the right cut and batch for me, that I rotated.
I agree with the Kate Middleton comment as well - yes, she has a classic, demure style, and shops at the high street stores as well as having designers donate things to her for publicity, but I'm fairly sure she also has an army of seamstresses to make sure everything is adjusted to fit her perfectly. Plus she has her own personal stylist and an army of people to make sure her hair and make up are always perfect.
Good luck!