Like most skills in life, focus is something we must develop and practice especially in this ADD world of ours where there's always some new shiny object trying to hook our attention. If you know it's a challenge for you - then the best way to start is to create an environment for yourself that is conducive to focus.
For example, creating a time-block calendar where you work in blocks of time where nothing is present to distract you from the task at hand. So if you want to write - start first thing in the morning and set one hour aside for that creative stream to flow BEFORE engaging your email or any other "left brain" type activities. Shut the phone, close the door, and just write whatever no matter what it is. Keep doing that at the designated time on a regular basis and you will train your brain to respond accordingly. Obviously apply this to whatever you want to focus on.
Using a time-block calendar is a great habit to put into practice. It cuts down on overwhelm, confusion, and all the other crap that prevents entrepreneurs from being as successful as they could be.