Another aspect that is also useful has been my experience of Pomodoro technique. At a job where we were practicing Pomodoro technique, I realized I don’t struggle with concentration anymore. The minimalistic explanation of the technique is to work on a task inside 45 minute intervals; then walk away from the computer for several minutes.
I find one of the additional benefits it has is that it gives you a means of quantifying your day. Often I can't remember exactly what I've been working on but knowing that I've completed x amounts of intervals leaves me way more satisfied at the end of the day and makes me feel much more confident that I've been productive.
I find it can also reveal how long some things take to complete. Rather than thinking 'uhh I couldn't even complete that task today' it can often lead to 'wow, I worked on that task for x intervals and it still wasn't complete - I definitely underestimated that one!'