I think this is all very personal, so you need to try a number of things and see what works for you. I have ADD, which comes with hyperfocus; I find it very hard to get focused on something, but when I do get there, I am totally in the zone, work just flows out of me, and I lose all track of time. But when it goes away, I am exhausted, spent, as if I'd just run a marathon.
So I have always had to manage my energy; I learned from very young that I just need to plan downtime into my day. It is hard; society says we need to go-go-go, and my own up-close-and-personal role model (mom) takes that to the extreme. But at some point I was just forced to realize that that is not me; that I need to come home and flop on the couch and watch TV or read a book or spend an afternoon cooking -- basically, anything that can turn off that part of the brain and activate a completely different one.
And btw, it's the "activate another one" that really matters. I find that my mind continues to race and fret and circle around whatever my current problem is, unless I give it something else to focus on. So even if I feel completely wiped, taking a nap doesn't help, because I don't get a break from that brain work. Cooking is perfect: I have to pay close enough attention to what I am doing so I don't forget ingredients or burn the food, but it requires zero higher-level thought and seems to use a different part of my brain. It's almost like a walking meditation.
So the main takeaway is really, IMO, know your limitations. If you are having trouble maintaining energy, do less of what requires that energy. And then find something else to do that you find mindless and refreshing -- a walk with your favorite music on, puttering in the garden, an hour or two with your favorite show on Netflix, etc. etc. etc.
And then be ready to adjust. As I get older, I find that it is harder to get into the zone, and that I cannot maintain it as long (of course, the fact that I am in a position now where other people are calling and emailing me all the time, and my kids are texting several times a day, doesn't help). So I have actually had to plan more downtime into my days, just to keep my energy reserves where I need them to be.
And yeah, I struggle with the guilt of doing less, with the embarrassment of admitting that I cannot keep up with my former self. But it's not going to do me any good in the end to keep denying what my body is telling me, you know? I can rage, rage against the dying of the light all I want, but my brain is still going to waterbug away from my work if I try to force it to do too much. Better to accept it and just deal with it.