Not everyone is cut out for long works. I know writers whose goal it is to work their way up to 100,000 words. Everything they do seems to conclude at 20 or 30K, and that’s the most they can concentrate on at once. If you’re struggling to do longer works, consider that it’s not something all writers are just able to do. It takes practice, pushing your limits, and developing the ability to hold an entire story in your mind at once.
I’m the author of three 300,000-word stories thus far (unpublished as of yet). It wasn’t intentional; my ideas are just too big. I kinda started off writing longer works and had to teach myself how to be more organized. I’ve had to work on telling shorter stories, without them growing into full novels, or even series! This doesn’t make me better, just different.
The education system puts too much emphasis on structure and technique. They have to, since they can’t grade creativity. Ignore them. Really, all you have to do is dive in and do it. Try things. Learn from others, but ultimately everyone finds what works for him or her. Practice doing what you want, and keep going even when it doesn’t feel like it’s working. Be prepared to fail, but you will learn from mistakes. Also, if you haven’t done so already, I do not recommend writing your favorite idea first. Odds are your first couple attempts will fail, and you do not want to spoil your best idea too early.
As for feedback, don’t expect any. Most readers cannot tell you what you want to know (does this make sense? is this character annoying? what do you think it means? are the visuals vivid enough?). Even after the work is published, you’re not likely to get much useful feedback along these lines. Most people know what they like and don’t like, but they do not know how to articulate it. That’s why you’re a writer and they’re not Write until you are satisfied with what you’ve done, then take any feedback you get as a gentle nudge to stop doing this or try doing more of that.
Feedback from editors is the most useful, I think, because editors (should!) know what writers need to hear. When you’re ready, start submitting stories for publication. Eventually some editor will tell you some things, and though it will hurt, it will make you a better writer.
And from a writer’s perspective, I recommend breaking your story up into chapters of some kind. Writing is about flow and creativity. Editing is about molding that flow into a structure other people can understand. Thinking about the story in blocks will help during the editing process. Readers also like chapter breaks because it gives them places to pause without feeling they’re stopping in the middle of something.
Have you read Stephen King’s “On Writing?” If not, I suggest you do, even if you’ve never read a Stephen King story. He recommends “don’t think, just write!” Plow through the first draft, wait a few months, then go back and read your own work and see how well you did. Editing is comparing what you wanted to write with what you actually wrote, and bringing the two closer together.
For me, my longer works grind to a half after the first 20-30k words in the first draft. Then I go back over the beginning, edit it, make sure it’s right, and finish the rest. Getting the foundation of a story correct is paramount. You don’t want to build an entire novel inside a plothole. Doing this has saved me countless rewrites in my later works.
Hope some if this is useful.