Best advice I've ever received:

"Never throw in the towel because of exhaustion. Throw it in because the model doesn't work."

Until you prove the model doesn't work, keep going.

I nearly gave up, without proving my model did not work, weeks before I raised $100,000 in funding. I'm glad I listened to this advice and kept pushing, even through the tough and discouraging times.

It's easier said than done, I know. I've been there. You know how to reach me if you need me.