Most of my stuff is electronic. I have setup my folders in OneNote, Outlook and Windows Explorer the same way.

#Environmental Stewardship
#Financial Sustainability
#Integrated Water Management
#Organisational Capability
#Relationships
#Service Delivery
@Done
@GTD
@Projects
@Reference
@Review
@Tickler
@Calendar Support

Those with a # are my areas of focus (I've only listed the work ones above, at home I have Social & Fun, Self, Sustainability) and obviously they have subfolders. I use AOF folders for support of habitual routine activity information (eg in financial I have notes on how to do the monthly accruals for our opex budgets).
I have a flat projects file like Gardener recommends. I did what you did first, found it didn't work, and listed to a DA webinar on the topic, he recommends a flat project folder. So there is one folder under @Projects for each project and they are not grouped by area of focus. However since I have separate work and home systems, by default only work projects are on my work laptop, home projects on home laptop.
If you try to sub categorise the projects it becomes hard especially at work, when a project fulfils several areas of focus.
I like OneNote for searching and storing info.
When I finish a project I move it to @Done for later reference.