Sum up
Here's how to focus:
Focus is a muscle: The more time you spend focusing, the better at it you will get. Don't give up.
Clear your head: Got concerns? Write them down. Make a plan for how to conquer them. Then get to work.
Location, location, location: Go where you know you'll get stuff done where there are no distractions.
Stop being reactive: Turn phone notifications off. No interruptions. It all starts and ends with you.
Get your sleep: Or you'll be dumb and impulsive. More sleep means better performance across the board.