Let me tell you that completing a book is no easy feat - getting started is just as daunting as getting to the finished line. For most writers, it is like slogging a heavy sack over ones shoulder, especially if it's your first attempt to write a book.
Some people write a book, do well, then fizzle out. There's a number of reasons for this: a lack of planning, self doubt, and a lack of motivation. There's one way to tackle this problem of not finishing this book, and it isn't by hiring a literary fairy Godmother to turn your book into a well-read, complete book. She might be able to turn pumpkins into carriages, but I am afraid you have to finish the 1st draft by hard graft yourself.
And one thing that will insist you in completing a book is by setting a goal. Trust me it works, and this is from a guy who spent four years writing a book; in most of those years I wasn't productive and I spent most of my free days lying in my bed waiting for Cleopatra to feed me grapes ( she didn't arrive by the way). I tried to get back into my book, but I just kept on saying to myself, "I'll start it in a few minutes," but I just kept putting it off, and ended up writing nothing.
But when I did, eventually, I got into a flow, and one thing that helped me to complete the 1st draft was by setting my word count and also booking a time with yourself to complete that word count. I work full time so it was important for me to put some time aside to fulfil a standard word count I set for myself. For example, I had chosen the mornings before work and after work in the evenings. I placed an hour aside for each allocated time, and told myself that I have got to write a certain number of words. In my case, each days word count was 900 words - give or take I had to reach that number. So I aimed for 400 in the mornings and 400 in the evenings, but most times I was going over that time. Morning time, especially after you've just got up, was the best time for me and I easily completed 900 to 1000, so by evening time I would write much more, notching up to 1,500-2000 in a day. Eventually my new word count standard became 1,400 words per day - and that's how I completed the first draft of my book.
So my advice is: set a standard word count number; start low, let's say with 500 words per day, and I'm sure you will supersede that number as you rattle along; whatever number is your everyday word count becomes your standard word count and hopefully that standard word count will be passé as you end up with a new standard word count.
What also helps is by silently setting the finish date of your 1st draft. For example, with my last book I promised myself that I would complete the 1st draft in four months time, so I was under a personal deadline. I did meet that deadline by the way. It's actually the first thing you should do before setting a word count per day or allocating time. It puts a little pressure on yourself - it's like a silent promise to get the job done. A challenge. If you don't meet that deadline, then you just stretch it to another month or so. Look at professional authors tied to publishers' contract where they have to meet the date - they can't afford to miss days, and most times they complete the deadline.
And you can goal set with any part of your writing, from concocting the idea to researching it and actually starting the book.
I'm not a so-called expert on writing but what I shared is what I feel helps, and that's by experience. I have completed two books and I am writing a third. But everyone has a different way of working but setting goals is a good way to reach that finished line.