Meltdown mode, help?

[sin_of_pride]

Okay, I need you guys. Thing is, I have a super-long story so far; while a large portion has to be re-written before it's good enough, there's already over 15.000 words written, closer to 20.000. This is a plot I've had since before last year's bigbang challenge, so I'm dying to complete it already. The problem is, despite the word count progress, I still have half the plot unwritten and it's driving me crazy. I think the final word count might end up being somewhere near 40.000 words (probably less) but the prospect is daunting. I don't know if I'll be able to write out everything that's still missing. The plot is a bit complicated, the characters are still giving me some trouble, so I'm worried I won't be able to pull all the threads together within the deadlines. I'd basically have to write like 20.000 words in 1 month (December).

/o\ Do I drop out? I don't want to, but I'm worried. Any hand-holding/advise/tough love will be appreciated right now.