I don't know if your book lends itself to this possibility, but have you thought about trying to sell/submit sections of it as part of a serial of stories or chapters? I was working on a nonfiction book a couple of years ago and was able to convince a magazine to let me write four articles based on the book (my query to the mag suggested eight, so I thought four was awesome.)
Anyway, I've read in a few places that if you can break down what you've written into discreet sections/stories/chapters, or even stand alone tales, and then try to get those published, and have the tagline say, excerpt from forthcoming book, "whatever the name of the book is," that kind of publication clip/credit can help convince an agent that there's a market for your work and that your work has been found "worthy" for lack of a better word, by another editor.
Whatever you do, if you believe it, really, don't give up. Maybe move on to other things that you can pursue simultaneously, but don't give up.