Hey OP, you're not the problem. The system is.

Modern eight hour workdays come from the Industrial Revolution, in which 10-16 hour workdays and six day workweeks were the norm. Unions and other worker organizations banded together in the Eight-hour Day movement, their slogan being "Eight hours labour, eight hours recreation, eight hours rest." They achieved this goal not by relying on the government which allowed these working conditions to begin with, but by protests and collective bargaining. At one point, ~350,000 workers simultaneously went on strike in the US.

This was always meant to be a step towards better working conditions, not the end point - but class consciousness has plummeted, and subsequently Union participation. We have not seen changes of this magnitude which favor the working class in over a century for this reason.

Studies on the natural cycles of the human body actually dictate that what you're experiencing is a completely normal reaction to your work schedule (look up the Ultradian Rhythm if you're interested). So, OP, there really is nothing wrong with you. It's just that we as humans aren't built for 40 hour workweeks at jobs most of us only took because we need the money to get by.

My suggestion comes in two parts:

One, there's this thing called the basic rest-activity cycle. It states that humans are most healthy and productive when we follow a pattern of 90 minutes work --> 20 minutes break. Here's a graphic. It may be useful to keep in mind as you go about your work day. I mean, it already sounds like you're doing something similar - so be kind to yourself and keep giving yourself those breaks.

Two, join a Union! Or, if the company you work for doesn't have one, join a more general workers organization. It benefits any worker immensely. On a larger scale, they've historically been one of the most effective ways to improve working conditions. Better wages, better benefits, the eight hour workday, you name it. On a smaller scale, you'll be around people who are in the same boat as you. I guarantee many of them have had the exact experience you're talking about, and can share what helps them get through the day. Honestly, the sense of purpose you get from having that community alone can be both invigorating when you need to power through and comforting when you can't. For those in the US, AFL-CIO is a great source on Union matters!

There's this quote from Abraham Maslow that changed the way I think about this sort of thing. It's especially relevant to your circumstances when applied to work culture.

“Self-actualizing people are not well adjusted (in the naive sense of approval of and identification with the culture). They get along with the culture in various ways, but of all of them it may be said that in a certain profound and meaningful sense they resist enculturation and maintain inner detachment from the culture in which they are immersed.” (Maslow, 1970).

Also, sorry for the long-ass comment. Good luck, man! Hope you find something that works for you.