As someone who has never been in this situation, some of the replies in this thread are very disappointing. They basically seem to amount to, "eh, go pound sand, quitter."

You've put in a ton of work during the hardest period for a startup. That's not worth nothing.

In opposition to these other people, I'd say maybe a frank but congenial discussion with your co-founder is in order. Something along the lines of being burnt out or not agreeing with the current direction of the company, those details don't matter so much. If think you'd want to ultimately fall on you wanting to "pass quietly into the night."

Sometimes people come up with solutions on their own that would surprise you. You just gotta not pigeon hole them into a particular type of response from the start.