I'll tell you what worked for me, but I'm afraid it's nothing you haven't heard before.
Take out a month to write the first draft. Tell yourself: in 30 days, I will either have a draft that I can then work with, or I will give up on writing altogether. If you're really desperate you will already be on the verge of quitting anyway, so it should be easy to believe this threat.
Stop researching. Now it's writing time, so write. And just write. Just get the idea out once and for all. Don't google to make sure you get the capital of Panama right, don't look up how that thing is called or who was governor in 1986. Just make stuff up, that's your job. You can always correct later. Actually you will do a lot of correcting later.
Stop outlining or otherwise planning the story beforehand. Having a vague feeling in the back of your mind of where you want the story to go is enough. Seeing just as far ahead on the road as the headlights shine is enough to get you there. Your only job at this point is to give birth to the thing, you'll get to raise it later. Your "perfect method", ritual, routine or whatever will organically develop itself as you finish more stuff. But you must finish stuff first. If you find that your story changed midway through, just start writing from there and backtrack when you're done. Make notes so you'll know what's going on, but keep them topical or you'll find yourself back down the rabbit hole of planning and outlining before you know it, and all momentum will be lost.
Write. Don't wait for the ideal time for writing and don't fret too much about writing for an hour everyday or anything like that-- just write every chance you get. In this stage, "discipline" is overrated. In fact, indulging on any preconceived ideas of what basically amounts to "project management", before you have learned to actually deliver, will just get in the way. So forget trying to stick to any method or routine you decided on abstractly. If any kind of deliberate management worked for you it would have worked already, but it clearly hasn't, so just write. Your particular version of discipline will manifest itself as you get loads of actual writing done. What's really important is that you keep the heat on. This was the mindset that changed everything for me. Even though I started with a "backup plan" of setting aside at least 30 minutes in the end of the day if I had not gotten any writing done that day, it soon proved to be unnecessary, as "sneaking in" writing here and there soon built up a body of work that surprised even me. I was even actually using writing to procrastinate on other things.
Don't read what you just wrote. Just read the last scene or page to get a sense of where you were and where you're going, then get back to writing. You're allowed to re-read it a few times to immerse yourself in the scene and get the imagination going, but make sure to only move forward. Which reminds me of another version of this same basic principle: don't edit while you write. You'll have plenty of time to get absolutely sick of editing, believe me, but now is not it.
Write smart. I don't know if this is actually "smart", but that's what I'm calling it. My first "first draft" ever was basically just a description of what I wanted to happen. "Mary calls John and he screens her call. He's not sure he wants to talk to her just yet. Maybe she shows up at his place? I don't know. Meanwhile..." It looked more like an outline than a piece of writing, but it felt like writing when I actually read the whole thing for the first time. I could see the scenes playing out in the back of my mind, I was flooded with ideas for scenes, dialogues, details, descriptions etc. You'll have plenty of time to come up with that sublime imagery, that perfect turn of phrase-- you just need to make something real first. It is much, much easier to change something that's already there than to create something out of thin air.
Follow these 6 easy steps and... congratulations! You now have a super shitty first draft. That's awesome, having a super shitty first draft is the first step to having a shitty second draft and so on. Sculptors get to choose a slab of marble that's already there, writers need to write theirs into existence. This is the purpose of the first draft. It gets better from there.