Man, this is exactly the issue I'm having with my big work. Had the idea since I was in high school, started and restarted countless times, have written segments of the middle, parts of prequels, and so on and so forth. It never goes anywhere.
I even have a big elaborate color-coded map of the world, that is actually pretty awesome as a map, and took me like two weeks to create, but it hasn't helped me write the story like I'd hoped.
After starting and stopping so many times, I realized there must be something wrong with the plot or the characters or something, because it just wasn't working...and still isn't. So, I set that one aside and focused on a single book, instead of a trilogy, for my first major undertaking. I'm almost finished with that book now, and all the time the trilogy simmers in the back of my head. I've struck a few good ideas that improve the plot, but there is still something fundamentally flawed with some aspect of the trilogy. I can't figure it out yet, but I plan to concentrate on that when I finish this stand-alone book.
Perhaps you should try that. If the book you're trying to write is, in your mind, your magnum opus, then you may be feeling some fear about not doing it justice. Set it aside, write a stand alone book. So far doing this has done two things for me:
1) I proved to myself that I can, indeed, write an entire book.
2) Through writing this stand-alone, my writing has improved A LOT. Now I'm not so afraid of disgracing the trilogy with shoddy writing.
3) Now that I've been through (nearly) the process of writing an entire novel, I have a better grasp of how to approach my next project.
It all just adds up to confidence I guess. The task of writing that gem of a novel or trilogy into which you've poured a good percentage of your youth is a daunting one. I would say try something of a smaller scale. I don't necessarily mean fewer pages, I mean a project that hasn't been sitting on a pedestal for a decade, accumulating all kinds of assumptions and expectations of you, its creator.
That's my advice. Best wishes on your project.
BLG