Take a pen and a blank sheet of paper.
Think about a topic that you are really passionate about.
You name it – fashion, cars, philosophy, photography… Turn your timer on and start writing on this topic until the moment you feel that your focus is starting to drift away. Even the question ”How long has it been?” also counts as a diversion of focus. Once you are finished, take a look at how much time has passed. You probably will be expecting that 2,5 or 10 minutes will have passed…but in reality – in case you have been super concentrated – the elapsed time will probably not be exceeding 30 to 40 seconds…
Can you bring that up over time?
Track you progress as you go along.