Boredom is the hallway between distraction and focus. Most people turn around in the hallway.

An unpleasant stretch sits between stopping the stimulation and starting to think, and it is a passage rather than a destination. Nothing in it is interesting, which is exactly why most people reach for the phone two minutes in and conclude that they cannot concentrate. The concentration was on the other side of the dull bit.

It helps enormously just to name it while it is happening. When you sit down and immediately feel like doing anything else, say to yourself that this is the hallway and it lasts about ten minutes. Then stay in it with nothing else available, and notice how reliably the door opens.