I have never been diagonised with ADHD. But, I am 66 and have had problems focusing on the task at hand all my life. I think that it is often related the number of projects you have on your to-do list. I find it helps to prioritize based on importance and ease of completion. If you tackle some of the easier problems everyday you will feel a sense of accomplishment especsially if you remember to check them off your list and update daily. Best results for me if this is done at night before going to bed. It helps to make you feel that the day was productive in some way even if you did not complete any of your major tasks. A wise man once said "If you make your bed every morning you will at least have that accomplishment and a comfortable bed to get into everynight."

Major tasks are best broken down into manageable chunks to allow for interim accomplishments and personal high fives. 

Meditation helps to bring the importance of what is on your mind into perspective.

And there is nothing like a 10 min power nap to get you recharged.