Writing is both the easiest and hardest thing I've ever done. I've mentioned my box of unfinished stories before... it sits on the floor of my office, gathering dust, while I wonder what to do with it. (Don't tell my husband.) I've come a long way, or at least I think I have, since then. Although, the "Writing" folder on my laptop also contains a few unfinished manuscripts.
Completing the first draft is a feat. The process feels a bit like climbing a mountain... and rolling back down a few times before you reach the top, if you reach the top. Lots of obstacles come between you and your goal. Self-doubt is a major one. Every story I write, I have to get over the "what was I thinking" hurdle before I can finish.
The First Draft is a Rough Draft
It's challenging to not edit while writing. I'm a bit of a perfectionist and constantly have to remind myself that the first draft is a rough draft, telling me the story. I use a few tools to keep me on track - a Save the Cat beat sheet, a plot worksheet (of a roller coaster ride), an editorial style sheet, a novel outline in Excel where I also track word counts (by chapter, act, and novel), and the Evernote app.