Don't beat yourself up about it. :( It was an honest mistake. She'll still get the mask she wants and you still got to make a cool new design, so it's still a win for everyone in the end. Treat it as a learning experience? I can really see how just making stuff to sell is easier than doing commissions (though I'm certainly happy you do them! :) ) as you get to express yourself however you want, and then somebody decides they'd like to give you money for it. That sounds way easier!