There are, of course, the thousand-and-one things that people (writers and wannabe writers) post or comment whenever a question like yours gets posed. The vast majority will comment "just keep writing" or "if you really wanted to write you would" and while there's certainly truth in that, there's MORE to the challenge of finishing a project as well. The simple fact that pushing or punishing yourself into writing, forcing it, hammering down, blah, blah, blah, alone isn't The Answer, as you already know. It's easy to say or think, "just do it!"
That being said, the single most important truth has more than one element of focus. The essentials include: TIME (real or perceived, available or lack thereof); Incentive/drive/self-discipline; and DESIRE/MOTIVATION.
More than anything, there is the very real need to WANT to write your story.
You are ambushed by feelings of doubt, insecurity—is it REALLY any good, or am I just kidding myself?—when you read or see so many others bragging and going off on how many thousands of words they accomplished today. You then ask yourself how you can possibly hammer out a thousand or more words a day, and if you don't, will you EVER finish a writing project.
There are many RIGHT and ten times as many WRONG ways to go at it.
But you first have to be excited about the story you want to tell.
So, you have a story you want to tell and you are truly excited about it. You WANT to get it written, and, maybe share it when you're done.
BUT, I do outline. By that, I mean I'll take this idea that has slammed its way into my creative imagination, and I'll use a journal or notebook to simply AT LEAST, come up with ideas and thoughts for each chapter. Those are my Roman numeral I, II, III, IV, etc. outline headers. I may, or not, go any further. I may just use this as a rough guide for moving along with a story that I hope will be something close to what I imagine it should be.
TIME: If you tell yourself that you WILL WRITE one hour a day. It can be 6 in the morning or 10 at night or anything before, after or between. You don't HAVE to stick with the clock, just KNOW in your creative writer's heart-of-hearts that no matter WHAT happens on any given day you WILL write for one hour. This is where it all starts and how you get into the mode that will help you finish a project.
You have written some notes to guide your effort, and perhaps a simple outline of the chapter events that you think ought to take place.
If you DO determine that you like and enjoy not only writing, but the story idea you want to give life to, if you determine to write a certain period of time each day, you are close to having the ingredients that will help you complete a project.
100 words a day, 5 days a week, 50 weeks a year, will generate a fantastic novella of 25,000 words. Awesomesauce!
Double that to 200 words each day, for one hour of serious writing, for 5 days a week and 50 weeks a year, and you are in NOVEL territory. And, there is nothing at all wrong with writing one novel a year of 50,000 or more words.