you sound gorgeous! I know you didn't ask about beauty stuff so much, but I would start there as it seems like you have a natural advantage (I would kill for a slim face despite putting on weight!) So, lots of water, shape your brows, bit of lipgloss (or full make-up, whatever you prefer!), make sure hair is glossy and cut nicely and make the most of looking striking as a starting point. Hold head up high. Then people won't look at the clothes so much (I find).
As for clothes, what is making you feel frumpy? Is it that stuff doesn't fit, doesn't suit you anymore? What's going wrong? I usually try to identify that first before then buying to fill the gap. If you like classic with an edge, then good jackets (leather, suede, darker denim, black) can help with that. Also get shoes right- go with what's in fashion (roughly) and invest in a couple of pairs, again with a bit of an edge. That way you can still wear some of the clothes you have but look more current and more you.
On here people do seem to focus on the one dress or one top, I've always found that it's the overall look especially accessories, shoes, jacket, bag, that make me feel current/frumpy and so that's why I always update those first and worry about the rest later. On Vinted though, I don't pay full price for stuff anymore, but you have to really identify what you want before you go on there, right down to brands, colours and shapes otherwise it's easy to get overwhelmed.