The biggest problem of being a solo developer is that there is no one else to drive documentation needs. Yes, documentation would be great for future you, but present you has no one (including future you) to chat with about the code to get an idea of what documentation would be useful. If you are forced to work just inside your head, then the result is going to be obvious and predictable.

And no, talking to the rubber ducky doesn't help.

I use "tired me". I think once or twice I've used "inebriated me".

In the beginning of my career I was much more diligent about this (but I also didn't write as readable code, so it was more necessary); the exercise was simple:

When I hit the stupid part of the day (the end of the day), I go back over code. When I don't understand something, I write down something about my confusion. In the morning, when I'm back to being smart, I convert all the confusion to comments and/or better code.

on the contrary, many good developers do know what to write. They understand what the software does, and why, and they are capable of explaining it. They know what decisions were made, and why they were chosen over other options. Above all, they know where the tricky bits are (because they solved them), which are, for the most part, going to be the tricky bits for anyone following.

Anything you talk to the rubber ducky about probably deserves writing down.