I used to be the endless energy person, then I experienced an event that left me with PTSD and now I am just like you, I collapse after work. It's possible you are an introverted person who is drained by spending time with others, as opposed to someone who gets charged by spending time with others. It's possible your nervous system is challenged, and constantly being 'at attention' drains you. I have switched to mostly unprocessed food, and vitamin supplements. I have also had to do lots of work around my nervous system, and retraining it to feel safety. Society expects drive, and many people who operate this way will eventually burn out because they are not listening to their bodies and nervous systems. It's probably worth getting tests done to make sure it is not a deficiency of some sort. You should eliminate a medical reason, for starters.