I use a plain text file, and I edit it in Emacs' fundamental mode. I have a key binding that goes to the end of the file and adds a timestamp and some separator

I use a plain text file, and I edit it in Emacs' fundamental mode. I have a key binding that goes to the end of the file and adds a timestamp and some separator characters, but that's the only customization. This single text file goes back 30+ years now, and it's survived all the fancy-pants TODO and notetaking software I've tried (a lot of which probably doesn't run any more). I think that long-term habits work best when their tooling is simple.