I get this way all the time and I think a big part of it is the 'grass is always greener' syndrome. The other manuscripts/storylines, etc. are always so tempting, always look better than what's actually in front of you. It's the thrill of starting something new rather than finishing, which is hard work.
In August of 2002 I joined a local RWA chapter and I talked with one of the women about crit groups, etc. I told her I had 3 books, all unfinished and she just said, 'you have to finish one." Sounded so simple, but she was right. Even though that book will NEVER see the light of day, I finished it and then I knew that I was capapble of finishing a book. It was a huge breakthrough for me and made it easier for me to finish others even when I hated everything I wrote. Nothing is ever really wasted - you could always lift stuff for another book.
So my advice is, even if something seems 'wrong' - keep going. Get to the end. Your first draft doesn't have to be 400 pages. A lot of writers I know have their first drafts end up somewhere around 200 pages and then they go back and add scenes and chapters, etc.
Good luck - you'll get there!!!!!