One thing I've noticed, and yes there are some big generalisations here, is that clothes from certain shops are on the frump scale. Therefore, if you are worried about looking frumpy, one solution could be to only shop in places where it is quite difficult to buy frumpy stuff, at least until you are more used to buying things you feel better in.
So, for me (5ft6, 40, size 14), big danger zones are Monsoon and Boden. Now, not all of their clothes are frumpy, but I reckon the odds are I am going to look frumpy in clothes. Too much going on, too much embellishment, too much pattern, dodgy lengths etc. it's too easy to make a mistake in those shops, and I pretty much always regret anything I buy from those shops.
However, when I go to shops like Cos and & Other Stories and Whistles, it is quite a lot harder (not impossibly obvs) to buy things that make me feel frumpy, just because they are coming from a different perspective than Monsoon/Boden (and M&S, Wallis etc) IME.
If you have things that make you feel good, maybe start going back to those shops more and seeing if you can replicate that feeling with other things. Conversely, do the clothes that make you feel frumpy all come from the same few shops? If so, avoid until you have refined your shopping until you can make better choices.
I write down all the clothes and shoes I buy in a notepad each year to try and get a better handle on my shopping. I notice time and time again that I just regret some things, whereas there are patterns of what I am glad I bought. That might help the process also.
I agree with a PP that it has little to do with age/size etc. Some people look frumpy at 15, whereas some people look amazing at 75.