I'm 25,000 words into the latest book I'm writing, which is roughly 35% of my word count goal and I'm in that headspace where I'm convinced I can't flesh this story out enough to make it! I don't really plan and outline extensively because I just don't write well with that much structure. I like to have a general plot idea for a book and then just go with it until it's done.
I'm enjoying what I'm writing now, though. I'm getting into the background of my main character and throwing embarrassing, uncomfortable moments at her because I find books are the most enjoyable when you put your characters through a lot of crap. Making their lives miserable makes for fun reactions and interesting plot twists, I believe.