I hate that we describe women as being frumpy so often nowadays. I do understand what you mean though about not feeling your best. Such small things can make a big difference I have found.
A few things I would try if I was you;
Try some different bras? Have a fitting somewhere like bravissimo? (not m&s)
Try and stick to slightly lower v neck tops or shirts. Round necks or higher necks can often look really matronly if not super slim. If you don't feel comfortable doing that, add the illusion of a v with a long necklace or scarf perhaps.
Look at the lines of your clothes and where the ends hit. For example short sleeves like in the dress you are wearing can often make you look bigger because it draws a line across a broader part of you. Same for looking at where your tops end.
For quick pick me ups I'd try a few accessories you really like. Nice earrings or a belt for example. These are often the things that really help pull an outfit together and I've found make me feel a bit more "lifted" if you know what I mean.
Don't buy anything you don't feel amazing in and want to wear straight away. Try looking around online and put together a pinterest board of stuff you like. Then see if you can put together a few nice outfits potentially with those items together.
Colour is really important. I agree that finding which colours work for you can make a huge difference. Don't be afraid to stand out and wear bright colours if that's what suits your colouring.
@ShebaQueen I did exactly the same adopting my own kind of uniform. It's evolved and changed over the years and seasons obviously, but it means I know exactly what works for me and can grab stuff from my wardrobe I know will go together and look good. 