Maintaining focus and productivity for long periods of time is a skill that can be trained, like any other. (More on that later, but spoiler alert, "feeling really guilty when you didn't work as hard as you wanted to" is not the best way to train this skill.)

What's the most optimal way to train this skill? If my maximum long-term sustainable pace isn't good enough to reach my quality target, do I have an unreasonable target?

This seems like an overlooked aspect of productivity. If your maximum productivity has a limit, no matter how motivated you are, you will end up eventually exhausting yourself by going past that limit. If your limit isn't huge, then this becomes a large inefficiency.

Most of the advice I've seen has been variations of "take a break". But I can play video games (with a strong focus) for long periods without needing a break. So is it possible to become intrinsically motivated enough to remove this productivity limit as with video games?

I think I've made good progress on replacing guilt and becoming more productive, but I'm constantly worrying about working myself to exhaustion and falling into a binge/recovery cycle. 

I couldn't find much elaboration on this in any of the later posts.