“Distraction-free” apps are ridiculous. They don’t help at all. In fact, if you have trouble focusing on the task on hand, then the problem isn’t the app. The problem is you.
Removing clutter from an app’s interface won’t stop you from flipping to Twitter every five minutes. A carefully chosen font won’t stop you from whipping your iPhone from your pocket every time a question pops into your head.
The problem is not even distraction. The problem is overstimulation, and it’s going to take some effort for you to fix it.