Turns out that I basically needed an assistant. Since I was building features for my cofounders, we decided that we should pair program (except they don't know how to program). So they act as the "feature manager" and come up to my apt for 2 hours a nights a week and we work together.
A few key points: the feature managers MUST have their thoughts detailed out. In order for me to crank out code, I need to know exactly what to do. I think there's a cognitive cost when you have to shift from implementation to design mode (and even testing mode). You'll be amazed how much it helps to push those responsibilities out of your brain (and yet still get fast feedback).
Doing this, we've gotten probably 4 months worth of work done in the last month, without me increasing hours. Seeing how efficiently I can implement features made my cofounders realize that it's worthwhile to pay me even 1/4 time, so that I can reduce hours elsewhere. Been a real game changer for our company.