A lot of writers seem to have trouble finishing what they've started. My advice is to set a weekly writing goal. Since I have a day job, the weekends are my main time to write. I like to set myself a goal of 1000 words per sitting, or, if I'm mainly making edits, work for at least an hour. It might not be as quick as writing full time, but if you keep to a regular schedule you'll reach the end!