I'm getting old just one day at the time, but I feel this one really came with age. For whatever reason, I've always liked to plan stuff. For a long time, I was much more a planner and organizer than a person who like to actually get things done. Obviously I had to adapt and evolve to have a chance in society :D
To-do lists, urgency/importance matrix, schedules... pretty much everything had to do with time, which makes sense because it's a limited resource. Guess what, some years pass by and we tend to find out that the energy, that previously seemed infinite and continuous, is now something that comes and goes. The peaks and valleys are now much more evident.
It may be hard to accept that you can't get something done even if you have the skill and the time. It's novelty, and not a good one.
How do I deal with it? Be aware, know that's how things work now, and LASER-FOCUS during the energy peaks. If the experience/skill is there, make use of it when the whole system is ready to perform at high level. Prioritize, do what needs do be done, Pareto's rule, Parkinson's law, whatever fits you. Get things done when you're productive and get the hell away (or engage in minor tasks) when you're not.
RMach