I think the reason most people fail to finish their first novel is a lack of motivation. This can be connected to not having a clear concept that they want to write about in their mind or to not having a reason to write in the first place. That is why I wrote two articles before this one about those very topics. However, sometimes people do both those stages right and still struggle.
If you think about the most strenuous physical exercise you have ever done - and how it took all your strength and still you had to keep going - then you will understand the mindset needed to write a book. You need that amount of determination, that amount of will.
When I write my books, I tend to set myself goals. I have a goal of 10,000 words a week when I'm writing a novel. My Fledgling Account novels have been over 100,000 words often, so the goal is merited, otherwise I would struggle to finish the book in what I regard as good time. For some people, that goal would be too depressing. They would need a smaller goal, but one that still keeps them progressing through their book. For other people, 10,000 words is nothing. 20,000 or 30,000 is closer to the mark. You have to decide for yourself what is manageable, but still challenging. Then you have to set your goal and stick to it.
Have a reward system. Once you meet your weekly goal, relax. I often break down the weekly goal into daily goals - 2000 words five days a week. Then I have some dark chocolate once I've done the daily write.
You will, of course, get to a point where the book feels like it is writing itself. For me, this normally happens once I get past the halfway mark. But there will be times when it feels like nothing short of agony. And that's natural. Expect that. Don't panic. Think of the end goal. Set your mind. Avoid distractions - shut down Twitter, Instagram, Facebook, or whatever does it for you. Promise yourself your relaxation time after you've met your goal. Treat it like an assignment. Have a deadline for yourself.
And then write. Best of success to you and your books!