Singleminded focus is hard.
Even if you can manage to focus on Task X to the exclusion of Y and Z, perhaps using tools like GTD, there are the higher-level questions of "what is the best way to do X?" and "should I be doing X at all?".
The distraction is clearest in programming, where deciding whether to implement method 1 or 2 can take longer than either and often longer than both. But it's there in other domains, too, a constant hesitation that prevents you from entering the state of zealous passion that really gets things done.
This is an example of the paradox of choice: because you have options, you're actually worse off than if you didn't. And it can be fought the same way: by removing those options.
In software, that's the sprint commitment. In relationships, it's marriage. In day-to-day life, that's the promise.
I hadn't realised before that a promise benefits not only the recipient, but the promissor as well. The promissor gets a reduction of cognitive load; they no longer have to think about whether the thing promised is a good idea, something they should commit to. This makes the promise more likely to be delivered not just out of honour and bloody-mindedness, but because delivery actually gets easier.
Which is why I'm spending my days exercising until my whole body aches, then holing up in a home office to sketch workflows, pull together game designs, and write code. To be honest, this sometimes seems like a pretty strange way to start a sabbatical.
But I don't have to think about any of that, because I said I would.