Honestly I really don't think there's a solution. You can try everything you can to get yourself into a state of focus but sometimes your brain just rebels against it. I've sort of accepted that there are things that can't be controlled for. If Ive been particularly unproductive for a period of time instead of beating myself up about it I just accept that I've wasted a shit load of time and move on. That's sort of removed the stress part at least.

From a macro perspective though I've gradually come to the view that the average amount of work a man can get done if he's really trying is more or less the same. You might see someone else able to work non stop but that might be because he's doing mindless repetitive tasks. For things requiring more brainpower it's impossible to run 100%. You also get to a point where physical quirks might be the limiting factor. I've had a few times times where I've forced myself to push on with something, pulling all nigjters and what not, and suffered physical and mental burnout consequently which averages out the "productive" period before. Consequently I've taken on a more zen approach to pushing myself, try my best but don't overdo it cause sometimes it might backfire.