Try to write the documentation for the feature from a end user's perspective first. Stick to the bare essentials and effectiveness for the user.
Then try to write a brief about how the feature can be implemented using the developer's perspective. List down the translation units (i.e., source files), libraries/executables, databases/filesystem structures/files that will be impacted.
There will be multiple ways in which you could implement the feature. While writing this down, you can list all the alternatives briefly and then note down which alternative you have chosen and the rationale for it.
Then try to write a brief about how you will test it. At this time, it can be your overall test strategy and some key test cases (not necessarily all).
Keep all of this brief (you are not writing War and Peace).
With all this prep work, your mind would start using its associative capabilities to suggest how you can proceed with the actual coding.
Hopefully writing things down will push my brain to think more, rather than using subconscious way to directly start coding.
I'll start writing docs from user's perspective and a few possible implemation variations.
Maybe once I start writing docs, I'll start getting better at knowing what is must to consider before jumping to implemation.