Reddit mentions of The Productivity Project: Accomplishing More by Managing Your Time, Attention, and Energy

Lots of good recommendations in this thread: Mindset, War of Art, Power of Habit, Deep Work are all great. My recent favorite is The Productivity Project by Chris Bailey. I listened to the audiobook and found it very engaging and helpful. I ended up having to put in digital bookmarks all over the place and then relisten at night so that I could make notes. Lots of practical ideas, several of which I've implemented and that have made a measurable impact. Here;s the beginning of the Amazon description:
Chris Bailey turned down lucrative job offers to pursue a lifelong dream—to spend a year performing a deep dive experiment into the pursuit of productivity, a subject he had been enamored with since he was a teenager. After obtaining his business degree, he created a blog to chronicle a year-long series of productivity experiments he conducted on himself, where he also continued his research and interviews with some of the world’s foremost experts, from Charles Duhigg to David Allen. Among the experiments that he tackled: Bailey went several weeks with getting by on little to no sleep; he cut out caffeine and sugar; he lived in total isolation for 10 days; he used his smartphone for just an hour a day for three months; he gained ten pounds of muscle mass; he stretched his work week to 90 hours; a late riser, he got up at 5:30 every morning for three months—all the while monitoring the impact of his experiments on the quality and quantity of his work. ....

I'm familiar with this problem too. There are a few things I have learned (and am still trying to better apply). My apologies if any of this is already familiar to you:

• Use your time in more productive ways by figuring out which times of the day you do best with different types of tasks. A lot of productivity experts encourage this. Most people are at their best with brain-work (creativity, problem-solving, etc.) in the first few hours of the day. If that's true for you, make your checklist the evening before so you can get right to it. Most people also are the least effective in the early afternoon, right after lunch, and tend to have that zoning out effect or even nod off. This is a good time to do simple tasks, like making calls, responding to emails, or filling out paperwork. Scheduling meetings around this time can be helpful too as engaging with others can keep your mind active, unless the meeting requires heavy group problem-solving or brainstorming. People who work long days like yours tend to get a second wind in the late afternoon/early evening, so that's the time to get back at those brain-work tasks, though you might choose ones that are slightly less demanding if you're not feeling super focused. This is also the time to review what you've done that day and determine what you need to do the next day. (*I'll come back to this in my last point.)
  
  
• When you start to feel like you're zoning out and can't refocus, take a break. Go for a walk for five minutes, drink or eat something cold (that will help energize you), do some stretches, etc. Avoid looking at a screen during these breaks. Your mind needs to do something different. If you're like me and need external input for your mental health, listen to an audiobook or podcast during your break so you're still not looking at a screen. Have a few selected ahead of time and ready to go before your day starts so you don't spend five minutes choosing a podcast episode. (Yes, I've done this too many times.) Getting sunlight is VERY helpful and one of the best ways to re-energize. If it's pouring rain or truly too cold to go out, try a blue light bulb or SADD light to simulate sunlight.
  
  
• I agree with others about taking regular breaks, and Pomodoro can be really helpful. It hasn't worked for me, however, as it takes me about twenty minutes to get into a groove with my work. If that's the case for you, try the rule of 52 and 17, in which you focus intently on work for 52 minutes, then take a break for 17 minutes. Alternatively, some people do better with sprints of 90–120 minutes with a 20–30 minute break. It's likely you'll get more done overall taking more regular breaks because you won't zone out as much or have to do as much revising of your work due to being burnt out when you did it.
  
  
• Plan your downtime activities ahead. Even if it's just chilling with Netflix, giving it a structure like your day can make it feel more rewarding. You get to tell yourself "I checked off season 2, episode 3," so you don't have to shift so far out of your work mindset that your mind resists. Also, doing something simple with your hands during your downtime can help you relax. Knit, crochet, doodle, color (huzzah for adult coloring books!), etc., even while watching TV.
  
  -*This isn't going to happen in the immediate future, as it will take its own work to implement and you don't have time during your current project. But setting up effective planning and checklists can make a huge difference. First, spend some time observing your energy levels and most effective times of the day. Chris Bailey lays out a great method for this and offers a spreadsheet for it in The Productivity Project, but you can also come up with your own ways to track it. Second, use those observations in your reviewing/planning process. There are a lot of different ways to plan your time, but I like to do this: on Sunday, I sketch out a general plan for the week, alotting only six items per day (though I do more if I get everything done) based on my master project/task list. Each morning I check in with myself to see how I'm feeling (I have a chronic illness) each and adjust my plan accordingly. Then I reevaluate the next day's plan at the end of the day based on what I've accomplished. That last step involves reviewing how things went, reassessing urgency if necessary, scheduling new tasks that have arisen, etc. I also stay attentive to the types of tasks when I'm doing my planning. Because of my illness, I associate very minute function levels with each task, but I think most people could just assign energy and focus requirements to each task and plan the tasks based on their focus cycles. Setting up some kind of planning process will help you focus on quality of work the next time you're in the midst of a long-term, demanding project.