That line about saving your week... it hits home. I used to feel guilty turning people down, so I'd always say yes. Then I'd be stuck doing things like dropping my own project to help someone else with a task that wasn't even due soon. Or saying yes to a "quick" favor that ate up my whole afternoon. Learning to say no to stuff like that... it was a game changer. My own work actually gets done now, and I'm less stressed.
How did you get past feeling guilty about it at first? I totally get that, I used to think saying yes to everything was just being a good coworker. But seeing stuff like what @seans73 said about Dave working seven days straight... it really shows how that "yes" habit just burns you out. Now I see that protecting my own time isn't being mean, it's just being smart. It's like you said, you can finally get your own work done without that background stress.