If you're having trouble finishing your book, there could be several reasons why. WARNING: this is not for the faint-of-heart.
1) You're writing about a subject and/or writing in a genre that's not familiar to you and requires so much "work" that the creative aspect of writing is smothered. This is especially likely if you're able to churn out short stories in other genres (or fanfiction) but your novel is something different that you're only writing because you think will really sell, rather than writing what you know. If you fit into this category, you may feel like you're holding your story together with bits of tape and string, and praying it won't fall apart on the agent's desk.
2) You don't want to finish because you secretly fear that your work will be rejected once you have. Conversely, you may also fear success and the responsibility/ publicity it may bring. Do you have a history of unfinished projects?
3) You're in love with the romantic notion of being a writer, but deep down you don't have skill or interest to put black ink on white paper. You likely fall into this category if you've ever queried an agent with an unfinished project. You may keep a nice personal journal and buy books about how to write a novel in a weekend, but this doesn't make you a writer. Writing is hard, lonely work and if you're not able to put in the time or effort, then you should probably find other creative outlets.