Well, I've passed that point in the writing process where I usually give up. That, in and of itself, is an accomplishment. By this time--that is, towards the end of the first draft if not before--is where I look myself in the mirror and ask the eternal question:
Can I see myself spending the next couple of years with these characters?
The answer, more often than not, is no. If "no" is my gut reaction, then I scrap the whole project. I know what you're saying: So you spent the whole last year writing this novel, and now you're just tossing it? What a waste of time!!
Au contraire. Any amount of writing you do is good experience. It keeps your brain limber and the fire of creativity stoked in your heart.
What it comes down to is this: I believe that if you become bored of your characters and/or story, it translates to the page. And the last thing you want to do is bore your audience. Now, if you're near the end of a first draft that's several hundred pages long, you should not make this decision lightly. If you've written that much material, chances are you cared about your story for a long time before something went haywire, and it's probably worth your time to reverse engineer the thing to see where it went off track.
But if you dig to the heart of your manuscript and still find a rotted pit where the heart's supposed to be, then you're probably wasting your time trying to extract it from the jaws of death.
What's the longest thing you've written but decided to scrap? Mine was 612 pages long. Beat that!
Oh, and by the way: Happy New Year!