I can definitely relate - but I have noticed that the bursts have got longer and less intense as I've got older.

It could be worse. I've noticed that the bursts have got shorter and less intense as I've got older!

Haha, yeah I was going to reply with the same. I used to work 12+ hours/day for a few months at a time. Now I struggle for anything more than a few hours a day for a few weeks at a time. AI has helped me recover some ground but it's still less output overall.