You remember a Saturday when you were 13 and you picked up a book and looked up four hours later. The room was dark, you hadn't moved, you weren't trying to focus. You were just *in it*. You don't have afternoons like that anymore.
You haven't lost the capacity. You've lost the conditions.
This distinction matters because the productivity industry has spent the last decade selling you tools to *manage* focus (timers, blockers, deep work schedules) under the assumption that the problem is interruptions. If you've tried those tools and they help around the edges but don't get you back to that Saturday afternoon, it's because you were never managing focus back then. You were absorbed. Those are different mechanisms with different requirements.
What's actually happening
The state you're remembering is what Csikszentmihalyi formalized as flow, characterized by complete absorption in an activity to the point that self-awareness, time perception, and external concerns drop out of consciousness (Csikszentmihalyi, 1990 - *Flow: The Psychology of Optimal Experience*). The conditions for flow are unforgiving and they don't include "willpower." They include:
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A clear goal at the level of the immediate action, not the project
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Immediate feedback on whether the action is working
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A challenge level closely matched to current skill
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And critically, a near-zero ambient demand on attention from anything else
The 13-year-old version of you had that fourth condition by default. There was no smartphone, no parallel set of conversations to maintain, no calendar feeding you reminders, no inbox refilling itself. Your attention had nothing to compete with the Harry Potter book, so the book won by default.
The adult version of you has 40+ active claims on attention at any moment. A focus blocker can mute notifications, but it can't unburden the *background residue* of pending obligations, and that residue is what keeps the flow state from forming. A 2018 study from Princeton found that even when phones are silent and out of reach, mental performance drops measurably for people who knew the phone was nearby (Ward et al., 2017 - "Brain Drain: The Mere Presence of One's Own Smartphone Reduces Available Cognitive Capacity"). Subtraction alone, even very thorough subtraction, doesn't reconstitute the conditions for absorption.