Then check out the Pomodoro technique if you haven’t done so! I’m always amazed how much I can get done in 25 minutes of focus.

I'm kinda like this and sometimes even more than 12 hours isn't enough to do a task that shouldn't require 4 hours (at least for me). It was always like this, but it started to worsen in the last few months. I was diagnosed with ADHD a few weeks ago and it improved a lot since I started taking meds.

This helped me when I used to dally, but assuming you are salaried, finishing your day in 50% less time means your effective hourly wage goes up as much

Definitely something akin to this for me too. My ability to concentrate has definitely fallen off since my younger years, precipitously since 2020.

That and I'm trying to re-learn Spanish to a useful degree¹ - better attentiveness to the task would help with that too.

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[1] I did pretty well at GCSE level three decades ago, then never used it again. I fancy spending time over there in future and don't want to be that guy who turns up and expect everyone to speak some English.

Ah you are not alone!