Try using a text editor that simulates a coding language. For example Notepad++
This will allow you to color code your thoughts logically as would a programmer, bookmark things, label, etc. Use this as your draft. This will allow maximum organization and perfection including organization of thoughts, chapters, topics, sub-topics etc.
Also check your keyboard, use something that is mechanical. I recommend a switch with Cherry MX Red, feels good and isn't too loud.
I too struggle with writing anxiety due to perfectionism (my palms get sweaty and I freeze up). Using a text editor to start writing has really helped me. Doubles as keeping me away from all of those word processor distractions and back to authoring :)