Ask HN: How to handle a boss/owner who has ADD task focus?

I work for a small startup where my boss and the owner are the same person. The issue is that whenever they think of a task or see a problem, that becomes #1 priority (despite the level of impact/severity, or if I am working on something else that was assigned).

This is very wearing, as I have to switch focus often to something that doesn't matter that much, and often my boss forgets about afterwards. Any discussion about prioritization he takes as challenging his authority, and becomes upset and dismissive. It feels like I have no input, and am just a machine to do tasks.

Is there a better way I should be dealing with this situation? Has anyone else been in the same situation and learned how to handle it better?