The short answer is switching. Pretty Progress cites Stanford University research showing that multitasking can reduce cognitive performance by up to 40%, and a computer makes switching effortless: one more tab, one more notification, one quick look at your phone. Willpower alone rarely beats a setup that rewards switching.
What helps is changing the setup. Sukha's founder, Steven P., describes opening his laptop to send an email and ending up researching whether medieval monks had better attention spans than we do. He tried coworking apps, music generators, Pomodoro timers and phone locks, and none stuck for more than a week. What he wanted was one button to press when work was hard and attention was drifting. Sukha is that button.