The reason you’re great at one-off compositions but can’t put a long-form comic or animation together to save your life isn’t because you’re a lousy artist, it’s because you’re a lousy project manager.
I know that doesn’t sound particularly positive, but you’d be astounded how many artists I’ve run into who are literally unaware that project management is a) a totally separate skill set from being Good At Art, and b) something you actually have to learn – they think that people are just intrinsically good or bad at doing long-form projects and that’s all there is to it.
Correctly identifying what it is that you suck at is the first step to improving!
this is pretty much exactly why all my chaptered fanfic is either cowritten or unfinished.