Hiraeth means-
A homesickness for a home you can't return to, or that never existed.This poem in a very indirect way, is meant to show that constant urge of wanting to get back to that parallel reality inside your head. Whenever you're unable to daydream, or try to quit it, you'll feel homesick. You'll miss those characters, those situations, those imaginary people who actually never existed.
You feel homesick for something that doesn't even exist.