* One of the best usage of your business partners might be taking over navigation of the sales cycle -if they're sold on the valueprop, they probably know where it can be sold successfully. If they can't sell it, that's a strong datapoint you need to consider re: viability.
* If the market gap is real, you might want to consider selling consulting solutions to that gap specifically to gain additional understanding to the market.
* As a fallback, you might be able to sell consulting/contracting on other fields until sales gets significantly better
[1] http://businessofsoftware.org/2013/02/gail-goodman-constant-...
It sounds like you have a sales problem. Find someone really good at sales and try to get them to join the effort. Sales cures all, and it'll give you the 9 to 5 income, and slightly more regular hours, that I'm guessing you'd like to see coming in. With sales you can delegate some more of the labor that's crushing you.
Also, if your product is stellar, you can probably raise serious venture capital and accomplish the same thing until you get sales up.
It's often said that people tend to give up right before they get to the finish line. I happen to agree with that. Being a successful entrepreneur usually involves being able to endure pain that other people can't or won't, making it possible to stay in the fight long enough to succeed.
The task is to balance this equation: hang on longer than you feel like you can, but not so long that you do permanent damage (monetary, mental, physical, relationship, whatever type) you can't recover from. Getting that right gives you the best shot at success and limits the risk of regret.
If you're really, really, really certain your heart isn't in it, then bail in a way that perhaps enables you to retain a stake in the venture long term, and try to leave it in good hands.
As PG suggests in one of his essays, I'm "digging a hole that's narrow and deep". I'm targeting a few thousands companies worldwide. The business area they're in is fast-growing and fast-changing and I suspect that they might be wary of adopting innovative solutions because they don't have clear ideas about where their own business is heading. Some don't even seem to have clear ideas about the prices of their own services and products (I tested this hypothesis by offering a simple, free tool to help them model their costs, and the feedback has been clear and positive).
I wouldn't say there's a risk of regret: I'll certainly regret it. Presently, apart from my mental exhaustion, I'm in pretty good shape, both physically and financially, but I need to set limits and draw lines. One of my main worries is that I cannot figure out what's not working, why I cannot get more clients on board and why it's taking so much time for them to go live. I developed the app at blazing speed and now the sales are going so slowly that it looks like nothing is moving and I find that unbearable. Maybe I'm just missing some entrepreneurial skills? I wish I knew which ones. Right now, I would just like to get rid of the whole thing, as irrational as it may seem.