It is mentally exhausting, and I think it's highly underappreciated just how exhausting it is to listen to/deal with other peoples' problems as a manager in any capacity - especially when you yourself are not able to vent (either because of privity or because no one else understands).

This is why delegation is so important. As an organization grows, it literally becomes impossible to manage everyone by yourself. But enlisting talented managers who understand these concepts helps to alleviate this issue.

To answer your final questions, maintaining energy requires deliberate planning of the energy you expend in other areas. For me, I rarely drink because alcohol has undefined impact on my energy levels (I get tired, and it's not like a video game that I can flip a switch and be awake and ready). But more importantly, organizing your activities between <energy consuming> and <energy restoring> has been particularly helpful for me. Once I took the time to understand what takes energy from me and what restores it, I was able to experiment with planning my days to ensure that I have enough energy to expend, while maximizing the energy I do expend.