Sometimes one more thing is the thing that' s going to push you over the edge. Sure...you want to be able to do that one more thing with the work ethic you usually apply to it -- but the umph just isn't there.
It's not that you no longer want the results .... of course you do. You're just not feeling it. One more thing is just one too many.
Solution? Let it go -- not forever. Just for right now. Set a deadline to start again (commit to it and stick to it). Just enough breathing space to dig yourself out of what ever immediate hole you're in. And then start again.
Sometimes, that's the best way to move yourself into better health.