I’m looking for a word — I’m sure I read it somewhere, but it’s not commonly used — which describes the situation where a theory (or something similar) makes sense in your own head but not to others.
A good example is a poem or something you wrote, something amateurish. It makes perfect sense to you, because you know what you are writing about, but others may find no meaning whatsoever in it.
Maybe the word you're trying to remember is intuitive.
For example, My proposal makes intuitive sense to me, and to other people in my field that I've shown it to, but I'm having trouble explaining it in a way the stupid reviewers will understand.
Consider idiosyncrasy:
A distinctive or peculiar feature or characteristic of a place or thing: (Oxford Dictionaries)
An idiosyncratic theory or poem is peculiar (and possibly exclusively comprehensible) to its author.
Could the word be esoteric?
restricted to or intended for an enlightened or initiated minority, esp because of abstruseness or obscurity: (Collins English Dictionary)
Solipsistic seems to be a match for the phenomenon you're describing, but I think it is hardly common use in conversational English. The term has several aspects, a philosophical one pertaining to "an extreme form of subjective idealism that denies that the human mind has any valid ground for believing in the existence of anything but itself" (Encylopedia Britannica). Another aspect is psychological, relating to a form of self-deception. You can use the Wikipedia article as a first step to enter this complex field.
Appropriate synonyms might be self-sustained or self-sustaining.
Consider resonate. For example:
Alice didn't enjoy the poem, but it resonated with me, as I understood the inside-jokes and background.
