Try Tim Powers suggestion of research until you find 20 things too cool not to use. Then you need to love the onion layers of characters and plot threads. You need to peel away slowly and live the tears.
Ditto Oz's great comments on characterization, plot threads, setting (world building), homework assignment of short stories to novel. Want a short cut? Read the Leonard Elmore short story and watch the Justified series or any of his other shorts that led to longer works. ( For a list check here: en.wikipedia.org/wiki/Elmore_Leonard ) Ditto Trboturtle w/his Game of Thrones suggestion. These are hows, but I'm intrigued by the whys behind it not working for you so far. You ask, "How do I think on a novel's scale?" You need to rewire your brain. It isn't easy. I especially like Superpsycho's, "...you need to let it live." I believe this to mean time to breathe.
Learn to love the deep layers. Learn to appreciate them and absorb them. Learn to slow down while digging deeper into your plots and characters than you ever have before.
Start by plotting longer shorts. Find the niftiest thing you wrote in your last short--and explore it. Write it. That done? Go back to your setting and find the coolest thing you wrote--and explore it. Write it. Find a relationship that's cool--and explore it. Write it.
Try to go for
depth not
length and then watch your word count. Aim for a 5000 word short, then a novella at maybe 20,000. As you learn to dig deeper, you'll find yourself naturally gravitating toward longer and longer works. If you're good at shorts, you already write well. Now you need to train yourself to enjoy the onion. Ah, "Become the Onion Lord!" Ha! Love GRR Martin.
Yeah, this is one of the problems I have, because I think in terms of what is absolutely necessary to tell this story. If a word is unnecessary, I cut it out. Adding in things not needed to serve the story's purpose feels utterly unnatural and wasteful and wrong.
Caution: Writing long still requires every word, sentence, and paragraph add to the story and be necessary just like in shorts, but you give yourself the latitude to explore in order to satisfy your reader.
And often I don't care about the B plots of many novels/shows because I feel like they're getting in the way.
Keep in mind that B plots aren't unnecessary. They are ways to explore the main plot from another angle. Sometimes, they are ways to build secondary characters for future work, but the plot will still intertwine like a natural skin and won't be unnecessary. In longer works, you need to include character beats for richness.
To be honest, Rechan, if you can't
care about those other onion layers then you probably will find the challenge impossible to achieve.
But I don't think they're as, well, clear as you're making it out. Take Storm Front for example. Aside from Morgan hassling Dresden (which accounted for what, 3 scenes?) the entire thing is focused on Finding out/Tracking Down/Taking Out the bad guy. There's no B plot.
I heartily disagree. If you assume Morgan was unnecessary, you probably feel the same about Murphy. What about Michael's kids? His wife? These are essential and necessary onion layers as he peeled away at his characters--always with an eye to the overall story. JB added tension to the story with those scenes with Morgan. What did you think about JB using the lightning storms as a secondary character? Unnecessary? Wrong. It added texture. It added tension. It added mood. It would have been shorter to just skip it, but he
choses to include these to enrich the storytelling and make it satisfying to readers.
You can't get texture into shorts like that. Good luck and keep us posted.
Anyone can report A to Z and that is a talent, but storytelling is an art form. There are subtleties, beats, interior thought (would you think Harry's interior thoughts are unnecessary?)... There is a deftness to drawing out the tale to satisfy a deep genetic need within human nature and that is storytelling. Anyway you get the idea.