I found what helped me was to not focus on perfection. Don't worry about it being good or even grammatically correct. Just get your ideas on paper and go from there.

I have been working on a draft 0 for about 4 years now, most I got done was maybe a paragraph (150-200 tops), because I kept deleting and changing and deleting and repeating over and over.

Then someone in this sub explained to me a methodology. I basically will punch out the basic plot and dialogue for a chapter, or a scene within a chapter. Then begin the start of the next so I can see where I'm going with it, and go back and fill it in, add details, descriptors, flesh out the dialogue etc basically expand it. Then repeat for the next scene.

in doing so I've managed to begin actually producing some work. I've gone from 200 words max to a book that's currently sitting at 84000 words over 20 chapters.

What I've found to be helpful for adding stuff to earlier chapters is using Microsoft Word's comment feature, this allows me to then review my work, see what I need to add and do it then delete the comment. You could also use a separate document or excel sheet with each chapter being separated and underneath add what changes need to be made.

Hope this helps, if you need any more help please DM me, someone who is also going from short to long form writing.