Hi Bakingtins,
I would recommend on focusing on a few non clothes things until you have finished your 12 week programme - you don't want to go out and buy lots of things, only for them to be too big for you a few weeks down the line.
To me, frumpyness isn't just about wearing clothes that fit you well, and that won't date too quickly. It's also more generally about "losing yourself" after putting your own needs last for so long.
First of all, find a hair cut or style that works for your face, but takes absolute minimum styling. You could speak to a hairdresser about how to make your hair work with it's own texture, and cuts that would be quick to style (e.g getting a few layers cut in, and simply drying it upside down for volume then giving it a comb). Maybe he might recommend growing it a little to give you more options. Whatever! Over done hair is far more ageing than hair that looks natural imo.
Avoid highlights like the devil. Don't listen to anyone who says otherwise. They are a bastard to maintain, expensive, difficult to find someone who does them well, and no matter how mousey someone's hair, it looks better healthy and mousey, than highlighted, slightly frazzled at the ends, with three months of roots as the person hasn't had time to get them re done. (disclaimer: I highlighted my hair for YEARS. I stopped over a year ago and have grown my hair back to natural colour. It is SO much better!!!)
Take a bit of care of yourself more generally - things like taking a multivitamin, eating well, plucking your eyebrows (HUGELY IMPORTANT) trying to get some decent sleep, using body cream (if you like it), doing sport, and using a good moisteriser/eye cream.
On that note, go to clinique or wherever and get an eye cream, moisteriser and face wash. And use them EVERY DAY!!.
These are all things that will make YOU feel better about yourself, and give you more confidence that you deserve some time dedicated to making yourself happy. If you don't think you are important enough to spend a few seconds each day putting on eye cream or whatever, then you probably won't think you are important enough to spend a bit of time to pull an outfit together.
Get some good underwear - I don't know if you are breastfeeding, but once your body has settled down, do go out and get some good quality, practical, yet pretty underwear. Not only to help clothes fit properly, but also just because. I have no idea why matching underwear feels so special, but it does. It's a mystery of life.
Buy just a couple of good quality make up products you know you will use - If your skin care is good, you shouldn't need much anyway. Something like a tinted moisteriser, an eyebrow pencil, lipbalm, cream concealer and cream blush. That should take you 90 seconds. Max. It's still very natural, but a little more polished.
So far you're already three quarters of the way out of frumpsville and we haven't even touched on clothes.