Research has been done that shows for whatever reason we seem to have a limited number of conscious decisions that we can make in a day before getting mentally exhausted. This is why a lot of people of power like to delegate a lot of their daily schedule, and more generally why sticking to a schedule comes easier for many people, because you don't have to expend effort to decide what you need to do next, reserving higher order thinking for things of priority.
Here's a nice, short, non factual article from a site filled with ads that says it differently.
https://www.entrepreneur.com/article/244395
I can't remember where I heard about this originally, but it's a big thing in managing stress and anxiety if you have to be active in tons of decisions a day, eg. parenting, and management.
Could be that writing music isn't as natural for you as gigging, and requires more hard decision making. For a performance, it's just playing.
On the other hand,
Beethoven was slightly alcoholic...