[powatom]

The easiest way to train yourself is to define strict boundaries for what you will achieve during a defined period of time.

If you have trouble focusing, the worst thing you can do is leave your attempts completely open-ended and vague. Define precisely what you want to achieve in an hour, and then give yourself an hour to do it.

If you don't get it done in an hour, stop and take a break, then give yourself another hour to finish it and start the next thing. Work in well defined blocks of time - the point is not to get good at 'doing lots of things within an hour', but 'sitting down and working straight for an hour'. Once this isn't a struggle for you, bump it up to an hour and a half, then two hours etc. Don't avoid breaks - your brain will continue to work problems out while you're off making a cup of tea or getting some air.

If you need practice, then you need something to practice towards. Give yourself boundaries.