Thank you for sharing your worries on this. It’s great to see people trying their hand at creating stories.
I’m not Boichi but one of his recurrent advices in the AskBoichi is “do not copy someone else’s drawing“.
That might be why you’re lacking motivation. You’re stuck trying to reproduce someone else’s really detailed artwork and hence can’t really mold it to your stories. It’s getting stiff because you can only reuse existing poses and event which might not fit what you’re trying to tell.
As Boichi said, “Draw your own and improve your style. It’s okay if the first times your drawing style may seem immature.”
Don’t try to rush to the end line, there is not shortcut in efforts. Remember that Boichi-sensei art sure seems great but it’s the result of over 20 years of continuously drawing.
It’s fine if it takes you a while to find your style, and it should.
I’d also recommend you to read this AskBoichi about writing a story, maybe you’re trying to rush too much and skipping a couple key steps in your writing process: AskBoichi about Manga 7 – Writing a Story
Now for the motivation part, do you have persons reading your stories? Getting feedback and writing for someone can give you that little something that can be missing.
It’s fine if it’s not family or friends, there exists many groups online in forums or facebook with aspiring writers, mangaka and such, where people reads and gives feedback.
This is only my personal opinion and I hope it helped you.
Don’t hesitate if you want to talk more about it. Also, don’t hesitate to ask directly this question about motivation to Boichi-sensei in the next AskBoichi about Manga. I’m sure that even someone like him must have days where motivation is lacking, but that’s also what it means to be a pro!