I think I know what's holding me back on my book: this seems like an impossibly huge editing job. Reading through the manuscript reveals that about 1/12 of the story actually holds together. The rest of it needs massive overhauls to make sense, and that much revamping is difficult to stomach. Especially since I've set the bizarre, arbitrary deadline of age 20 for me to sell a novel.
I need to divide it into small chunks, and then get it done.