My marriage shares some similarities with yours and Xausted's, and I was deeply unhappy for a long time, but then I started to focus on my own stuff: mainly my career. The kids and I are going to be apart from dh for a while (b/c of my work) and I reckon the absence will "make the heart grow fonder." Is there a way you can get away for a while? That's not feasible for most people, but thought I'd ask.
Unfortunately, the heart-flipping thing eludes a lot of people, even in the best of marriages, and I think we as a culture suffer from unrealistic and romanticised notions of marriage. Yes, lots of people CLAIM to feel passion for their spouses 20 yrs down the line, but I have to wonder if that's not just just a bit of crowing from image-conscious types. I personally think a lot of people are less than honest about the dark sides of their marriages. But because I think honesty about difficult issues is a good thing, and helps move society forward, I'm not afraid to talk frankly about my disappointments with marriage. (Remember way back when, when mums couldn't admit their ambivalent feeling about their children?)
I think you need to ask yourself if you'd rather be alone. Because, of course, there's no guarantee you'll find someone "out there" either, at least not immediately. And if you would actually rather be on your own, then there's no point in pursuing with the marriage IMO, not for the children, not for anything.