Natalie, (I'll call you by your name since everybody here does, hope I don't offend!), first off, I so feel the pain you are in now. I commend you for writing it out here, for starting this post. It has generated so many valuable insightful ideas from members. I wish for you and your DH to grow from this. Relationships are always in evolution, with ups and downs.
I so agree with Fruitful's analysis of SYC.
And about buying new clothes: No, it is not frivolous, it is not superficial, and it is not something we can cut ourselves from completely without consequences. It is an essential part of loving ourselves.
Clothes and accessories cannot be viewed and cut out of a budget the way new video games can. It's not the same. A video game is pure pleasure. Dressing and self care is pleasure AND necessity. Take it away, and you are also taking away part of an ongoing PROCESS within you, a flame that has to be kept alive, even if tiny. He might not see the necessity of it, but then again, he is not a woman in 2012.
Not to mention that trying to simply cut yourself out of fashion and new clothes only brings on more expenses later on. SYC can only be done for a while before it crumbles and everything has to be started from scratch.
There are many ways to value yourself but in this society we live in, where image is so omnipresent, where we meet a lot of new people for a very short time on a regular basis, clothing and self presentation IS crucial. Much more crucial than a 100 yrs ago when pple stayed mostly among the same group daily.
But most importantly, we are fighting off daily oncoming difficult messages forcing us to compare ourselves with impossible, fictitious models: 14 yr old girls with perfect skin advertising skincare, Photoshop doctored pictures of impossibly thin, impossibly long-legged beings, supposedly "curved" women appearing very slim, Real Housewives of (your city here) throwing money out the window, throwing tantrums too, while fully dressed as if going to a Gala at 10 in the morning. And the list goes on.
You have to be a saint or a very special type not to be affected by this, even remotely. And there's nowhere to hide! Everywhere you go it's there. Believe me, even in third world remote countries its omnipresence torments women there just like here.
It is paramount for us to find a response in order to claim ourselves back from this. We in the year 2012 have to do so much more in that regards than our grand-mothers, than our mothers even. The idea that shopping for clothes is frivolous and unnecessary is something that stems in our grand-mothers' era. It is not true any more. It is a wrong idea in that it shuts the door for us to any fighting back to the destructive messages we are bombarded with.
You have crafted a method and were finishing a buying-returning project. Obviously you are responding to this intelligently. But your DH couldn't see any of it, because he probably thinks buying new clothes is equal to buying a new video game.
It's not.
So sit down with DH and rethink this whole SYC business. Explain things in detail to him. He has to understand there is more to it. Also revisit your earlier promise. Own it, yes, take responsibility for what you did. But at the same time, don't beat yourself in the head too much with it either: know that there are other factors that make SYC an almost impossible task. The mistake here was maybe to set the impossible goal.
A last idea: when discussing with DH, propose concrete solutions to bring the cost down of this necessary expense: sell clothes to second-had shops, buy online when it's less expensive, visit places where clothes are cheaper. Your DH will love concrete ideas.
Sorry for such a long post!