tl;dr: I use Dynalist, a free webpage/app that allows you to generate collapsible outlines, to keep a to-do checklist and journal bullet points for every day. I like this because things that don't get done can just be dragged into a new day, and because bullet points for journaling lowers the mental energy required to keep up with it. This system is highly flexible, goes with me everywhere, and lets me keep almost everything in one place. (I use Google Calendar for things that need to be scheduled more than two weeks out.)
( Setting the scene: )
Then, in early January, I read Tobias Buckell's post on bullet journaling, which made it sound sensible and appealing. After a little more reading (I noted down this Buzzfeed post), I found a recommendation for Workflowy as an electronic bullet journal—necessary because I will not carry paper with me everywhere and I need to have access to my to-do list at all times—the examples were way more complicated than I needed, but did show that it could be very powerful and customizable.
I stopped using Workflowy at the end of July for two reasons: first, the mobile app was not good, it was way too easy to drag stuff around by accident, but hard to move it on purpose; second, it only lets you keep a single (infinitely long and collapsible) outline, which was cramping my reference-note-keeping-consolidation.
Hence: Dynalist. Which is free, and which has an Android app in beta which is much more functional, and which lets you have multiple documents. (It also has a Workflowy import.) The paid version is a smidge expensive ($8/month billed annually), but I haven't felt the need for it; I might, however, end up signing up anyway just to toss them some money. I use a pinned Chrome tab (like so) on desktop, because I didn't realize it had a Windows desktop app in beta until I just checked.
( Here's how I've adapted the general bullet journal idea to a collapsible outline app: )
I've sometimes gotten a few days behind, but never more than a few, and I've always gone back and filled in at least a point or two about the day. And I started on January 9, so that's really pretty darn good. It hasn't been a magic (heh) bullet about getting things done, but it has reduced the amount of effort I put into keeping track of things, which leaves more energy for actually doing them. So on the whole, I'm pretty happy with it; and in case it sounds useful to you, here it is.