You just gotta write/ create a story you want to read. There are going to be people who won't like your story, who will criticize you, and beat you down but you just gotta keep on going. I would recommend watching youtube videos on writing like Abbie Emmons or other writers who write novels. After youg et a grasp on writing then switch over to learning how to draw story boards or manga. In my personal opinion the art is important yes, but if the story sucks, I'm not reading it.
Sketchbook thing: I've never once filled a sketchbook...that's a lie I just finished one, but it took years for me to finish it. Sketchbooks are just for sketching and getting your ideas on paper without commitment. If you're so focused on trying to finish a sketchbook, you'll never start your projects. You can sketch while doing it or after or whenever. There's no rule to complete it.
And to make art for yourself is just that. Draw. None of my sketches from my sketchbook sees the internet. Why? Because they don't need to and two because social media is a hassle, and I don't want to constantly upload just to indulge a few followers. I have stories written that have never been published because I like writing them. It satisfies an itch. I have three comics I want to work on LOO, TDD, and POB and including now I'm brainstorming them. That's part of the creative process. You just keep thinking of new things.
I used to go through the same thing you're going through. I was jealous and kept comparing myself. Quit drawing a few times and then continued and quit and continued and then never started my project. I got good at drawing and writing, but I never finished anything. Now, I'm approaching my thirties, and I just want to get it all out there before I croak. You can take hiatus's (that's what a lot of popular webtoon artists do) and just have time to yourself. You don't even have to publish it immediately. You can publish like next year after you have enough backlog and you're enjoying yourself. My story isn't even published yet and I'm just enjoying the ride.
That's how you create for yourself. Creating stories that you want to read and not giving a fuck about anyone else. Enjoying every moment your story comes together. The progress your art is making, the stress from not doing something correctly, the artistic burnout and just playing a video game or going on an episode binge and coming back to the world you created because real life sucks and your babies are your babies.
Creating for other people is going to get old fast and then you'll never be happy with your project.
That will inevitably part of the creative process.
Like mine for instance:
A cute story about a young demi-god hunting down ghosts and monsters with the god of war

to a romance about a demi-god meeting the god of war and being a fated pair while navigating the mortal realm and fighting against those who are against them.

Yeah, it'll change. It's surprising but in the end some changes are for the better.