Why is "own self" separated into two words, while other reflexive pronouns like "yourself" aren't?
Here is an example:
I am at war with my own self and it has become a constant struggle to wake up and face the day.
The real reason is "just because": it's how we write it, and that's just the way it is. But you might want something more than that; if so I hope the explanation below will be of some use.
We spell "yourself" as one word to match the spelling of other words like it, like "himself" and "themselves", that act as single words grammatically: these words cannot be split up, and they are formed in specific ways for each one that have to be memorized. (For example, for no obvious reason, we say "himself" and not "hisself".)
The phrase "own self", is written as two words because it does not behave as a single word: we can stick other words in between, like in the phrase "your own true self". In contrast, we can't say something like *"him true self".
You might notice, though, that we can say "my true self"; this is because "my" by itself is a possessive pronoun in English, unlike "him". So in principle, it might seem to make sense to use the spellings "my self" and "your self". But in practice, it is considered an error to use these instead of "myself" and "yourself". That is why I said earlier that part of the reason is "just because".
Google Ngram Viewer (We see that the phrase "own true self" exists, but *"him true self" does not exist.)