I do not aim to code for 8 hours per day in a professional context. I know very few people who can really honestly do 8 hours a day of "thinking" work on a regular basis. It may be possible in some cases: perhaps on a new project where there is a lot of boilerplate code to write and you are free to make design choices as you see fit. However, most of the time when writing code in a existing large complex code base I find that 5-6 hours a day is a very productive day for me.

I work from home I work in bursts of 2 hours. When I get stuck/bored (either because I don't have any more focus left or I'm stuck on a tricky problem) I will read, shower, go for a walk, or take a nap. When I come back to the problem, I generally find I am able to resume work.

I find that I get as much done if not more than my coworkers who must be in the office 8 hours a day. I also graduated from my PhD using similar tactics, while many of my classmates seem to LIVE at school.

My point is just that the 8+ hour work day may be possible for some types of tasks, but for real "thinking" work I don't think it is a good goal. You are burned out because you are trying to do achieve an unreasonable goal. Redefine your goals so you can succeed and be happy.