I've been there. Just remember that the best indicator of future success in business is having tried before--even if you failed. In the valley, it's a truism that VCs don't mind seeing a couple of failed companies in your past, as long as it's clear that you learned something from them. There's even some evidence that the same investors that gave you money last time (and you lost) are actually more likely to invest in your new company than someone they've just met who has no record of failure. Weird, huh? Failure is practically a virtue in high risk business like tech startups.

Take some time off, work on something else that's a little less stressful (I did part-time contract work for a couple of companies that I enjoyed working with--contract work pays well, and gives you lots of time off to relax), play some video games, talk to girls/boys, learn a new language or to play a musical instrument, read good books, enjoy time with your family/pets/friends.

In six months, or a year, you'll feel rested and ready to dig in again. It'll actually come sooner if you have an idea that you really love. That is what happened for me, though I had some downtime before finally giving up on my previous business where I'd stopped selling the products that I'd always sold and started slowly evolving the business into something else--but I was accepting no new clients, so the workload was quite low.