I totally agree with you here. I have timesheets going back several years. None of these timesheets showed that I have ever been effective working more than 30 hours a week. Just as you found, I can only do 3 hours or so on a single project - after that, I have to switch to something else.

My ideal work day is something like 3 hours freelance writing, 2 hours Medium. Sometimes more, sometimes less, depending on what other things come up. These days, I think I'm getting more exhausted so those 2 hours Medium gets cut down to one or none sometimes.