The thing I don't get is while it had been over 5 years since I took a real holiday I wasn't working the same number of hours as my work mates. I had a brief peak of 80 hours a week + other commitments. Then I just didn't really recover. Took my first long holiday and my productivity is coming back.
Recently I've managed to turn things around for myself by nearly eliminating carbohydrates from my diet during a short break from work (which also resulted in me eating less). I can feel the change. It's allowed me to lose some weight and start exercising again, so hopefully I can maintain this.
It was an easy change to make, really; before, feeling hungry was a painful state that I had to remedy. Now, it's just my body telling me I should probably eat. My goal for now is to burn some fat, so I choose to ignore the feeling and maintain some calorie deficiency. I do try not to starve myself though.
I think if anyone's suffering from similar trouble, experimenting with your diet is an easy and low-effort way to hopefully get started on recovery.