Hi! I want to write fantasy or science fiction but I'm no good at world building, I always get hung up in the details. Should I give it up and write a believable story in an already existing setting (which isn't the kind of story I'm used to reading) or go ahead without much planning? Feels daunting.
  Aha! I’ve been waiting for this question to pop up eventually and I’m glad you’ve asked it! My friend, you have what we in the industry refer to as World-Builder’s Disease. This is the very irritating habit of feeling like you have to know every tiny detail about your story before you can even think about writing any of it!
  It’s okay because I have it too! In fact, you’ll be hard-pressed to find any fantasy or science fiction writer out there who doesn’t have it. Before I get around to answering, I want to help you as best I possibly can by giving you a link to the fantastic 2016 University Lecture by Brandon Sanderson, as one of his lessons covers this in a way I cannot hope to replicate called “The Iceberg”! So here! (Don’t be intimidated by the length of the video, it’s just a small portion!)
  But now, the written answer! We like to see excessive amounts of world building as a necessary step in writing a fantasy novel, but honestly? It’s toxic. Not only does it take up huge amounts of time but it also restricts you and your creativity! It changes the course of the story you want to tell because you’ve already pre-determined everything. Your best bet is to go into the story without too much planning, leave everything loose and give your story some wriggle room.
  Let’s say, for an example, your Hero enters a town (for which you may or may not have a name for yet) and you want something to happen. You want some action. So throw some assassins in there to ambush him out of nowhere! You don’t need to worry about who sent them or why - the important part is that you make it exciting for the reader! The interesting part here is that you do the world-building after you’ve written the scenario!  It sounds weird but hear me out, because now you go back to that scene and think “Wow, this is a very weak scene because it leaves so many questions unanswered. Let’s answer them!” Then you possibly explain that this town is actually run by a sect of organised criminal cultists rife with assassins and killers, and whoever is in control of this town sees the Hero’s arrival as a threat to his stability, therefore sending assassins to remove him!
  Now this isn’t to say that you don’t need important details for your story before starting but you’ll be surprised how little you actually need. It just means that instead of asking yourself what it is about dragons that makes them so keen on stealing away damsels in distress, just do it and have fun with it, and during the editing phase explain it then! Otherwise, it’s like trying to make an ice sculpture - you have to start off with a rough shape before you can decide where to cut the details!
  I hope this answer (and the link) has helped somehow! If you have anymore questions or feel l you need more explanation, do not hesitate to ask! Good luck on combating your world-building and I look forward to seeing that book out there in the future! Happy writing!