I have thought about this, though not recently - perhaps like adjusting to a sort of disability, or maybe like adjusting your mind to accept severe restrictions, as an astronaut or a submariner might do? Shutting down parts of yourself in order to fit into a confined space...

I wonder also if language has anything to do with it, in the sense that language is supposed to dictate, to some extent, how people think and react? A person that can sing in the Ainulindalë seems like he should be a different person to someone whose expression is tied down into the languages of Middle-earth?

I'm not sure if the yearning is necessarily a scary 'implanted by external forces' thing, as it is with the Elves. You'd think they would naturally be homesick, it seems like the obvious emotion, even if they could nto remember quite what for...

Intriguing basis for story!