Ugh, I know how you feel. I've stopped writing two stories, one of which I need to finish because it's almost 60 chapters and I've been working on it for months. Bleh. I start and stop and love and hate with writing.
This used to happen to me every time I started a story, so now since I'm currently writing a time travel story I try to listen to music that will inspire me or look at pictures of the particular landscape I'm placing my characters in and it seemed to help a lot this time around =)
In the past, when this has happened to me, I push through it, but in draft form, and I try to take at least two trails. Then, I set them aside for a day or so, and do something that really inspires me (Listening to music is my thing), and then come back and look at what I pushed out.
Sometimes, I look at one of the paths and actually like it, I just have to make it more inspired, you knw? And sometimes, I like neither.
In which case, I give myself a break for say, a wekk, and I go back over my story again and try to see where I lost the inspiration and what I can change so that the story is exciting for me to write again.
Sometimes, I make a major change in the process, and sometimes it's as simple as removing a really dull chapter and reworking from there.