I started a writing a few books and lost direction, so I just stopped. How do I build consistency and finish writing a book?

You’re still learning your craft, I suspect. If your dream is to write fiction you need to learn the basic skills, like creating characters with realistic dialogue.

When I was starting out, I took some cheap, short screenwriting courses that really transformed my writing. The learning in those courses is in doing the exercises. Later on, I taught a course of my own, and I would give the class exercises where they had to create characters without telling them that that was the point of the exercise. In a class environment (though I also gave them exercises to do at home) you can learn those essential skills without simultaneously trying to write an entire book from scratch.

The other point is that you need an enormous amount of self-belief and you need to persevere.