1) Be ruthless about what you need to accomplish. This means choosing what to "do poorly" at. Sure everything is "important" but it's time to secretly throw a few things overboard.
2) Do whatever you can to optimize your wellbeing. Go to bed early and sleep enough. (1/3 dose of melatonin if you have trouble falling asleep.) Plan simple healthy meals that don't require much thinking or prep. A media fast is really good. Try to throw in a tiny bit of exercise (20 minutes) and 10 minutes of meditation here and there. (ex: when you arrive at the office)
3) Do the work in cycles so you get time to think between bursts. As we get older, the work wears us down so I treat screen time as "toxic exposure" and set a daily maximum. That doesn't mean you have to stop working...Just step away from the desk and visualize what you're going to do. Then get in there and work quickly and efficiently and get out and plan the next burst.
Bonus: In a real pinch, you need to work every day of the week. Though extra cycles and wellbeing make this pretty easy. If you're working too hard, the weekends are necessary to recover.