I think it varies by creator, what gets them through a project. For me, I have to love the characters and care very deeply about what happens to them in order to have staying power, but I know some people are driven more by the need to explore an idea that really fascinates them or by the desire to draw/write something they find aesthetically pleasing.
I do think there's a chance you might just be exhausted from your last project. I always take a break between bigger undertakings, where I focus more on reading/family time/whatever else for a month or so before diving back into another mammoth undertaking. I may work some during that time, but I try to ease the pressure.
Also, I think there's something to be said for giving up on the idea of finding "the one" story idea to commit to. My current comic has already become my longest, most intense art undertaking I've ever done, and I kind of started it on a whim. But that's part of what made the idea work for me - I actually decided, going in, that I didn't care if this comic was a complete and utter failure or if I dropped it in the middle and never returned to it. And for me, that took off a lot of pressure. It made it so that I could just produce something, without getting stuck in my own head over whether or not it was good. I really had to give myself permission to create crap I hated in order to make something I love.