Something you could try as an experiment - start with just one outcome you'd like to mark off as "done".
Start with a single sheet of paper and dump every idea, phrase, thought, etc. about this one single thing out of your mind and onto this page. Go for wild success of "complete". Do you have all the data required to move forward? Do you have all the questions or concerns noted? Make the big list on this single thing.
This page is now your Action Support.
Next, take a second page write the successful outcome of this one thing and label the page "Projects". When can you really mark this thing off as "done"?
Then, take a third page and write one (ONE) next physical, visible action to move the thing forward on a third page called "Next Actions" or use the name of a context - e.g. "Computer". Make sure it's the very next thing even if it's, "Open Google re: Project XYZ, search the web for ideas ..."
In my mind those three pieces of paper now constitute a GTD system.
The rest is adding item #2, #3, etc. Add a Someday/Maybe, Calendar, Reference files and a labeler as you go.
How did you capture that thought?
Inability is an abstract thing involving comparison with alternate universes; it cannot be experienced.