Like Chelsea said, I think it’s super cool that you’re self-evaluating and wanting to take the right steps forward!
This actually happened to me these past couple of years, where I had a historical-fantasy that was too large-scale for me to handle. I started it, revised the premise, and then started again way too many times. Right now I’ve moved on to writing a contemporary fantasy with lower expectations for myself, and I’m nearly halfway through and enjoying myself a lot 🙂
I think that if you’re sure you want to keep this idea around, you can do worldbuilding, character development, plot and even write short stories on it, to kind of get a feel for the background work you’re doing. If your goal towards improvement involves writing consistently, and you find inspiration in an easier project (as I did) then go for it! You’re not betraying your first, grand story idea. You’re just improving yourself so you can get there.
If part of your concern is that you’ve never written such a large work before and you want resources on planning the plot, I recommend what KM Weiland has on her site: https://www.helpingwritersbecomeauthors.com/. It’s helped me know what to aim for in a novel (or just a story!) in general and the plot points that always make a good story.
INFJ // The light shines in the darkness, and the darkness has not overcome it. (John 1:5)