A journal field attached to each trade
One text box per trade, filled in after the fact. What I was thinking, what I noticed, whether I nearly intervened.
The reason it should live with the trade rather than in a separate document is that a separate document never gets reread. If the note is right there when I review the trade list, I'd actually use it — and after a few months I'd have my reasoning attached to outcomes, which is the thing I currently have to reconstruct.
Even better if it's searchable and exportable.