I find that reducing the number of decision making events by relying on a routine helps. When and where are you endlessly watching Youtube or Netflix? Is it the first thing in the morning after you grab your phone or is it happening at your desk. Depending on the answer, you want to establish a routine so you don't have to think about what you want to do next.

Let's say it's your phone first thing in the morning that is causing this problem. You could put your phone and charger far away from your bed. Then establish a sequence of actions that take you from bed to desk, without getting anywhere near your phone.

This is not an all-or-nothing method, where you fail even if you miss one step in the sequence in your routine. You start by making one or two adjustments and then build on your success.

I start a timer on my phone and run it up to ~ 8 hours to mark the end of the day. While the timer is running, I don't do anything too leisurely (e.g., playing video games). If I want/need to do something unrelated to work, I stop the timer.

The idea isn't to work exactly 8 hours per day. It's to get a sense of how much of my time is going towards work-related activities.

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Recognizing that you cannot produce value while multi-tasking was enough for me.

You're just doing surface-level work if you're able to watch Youtube videos.