I can't remember what it is called, but in essence it is rhyming ideas or concepts instead of words. repeating a very simular idea, altered a little in language, to further emphasize it.
prehaps i have utterly failed to convey what i mean but i recognize you doing it here. What This poem conveys, although deep, is fairly simple. You repeat it with small changes that alter the meaning slightly yet still resemble your previous lines. This is a technique i have not been to succsessful with but you use it nicely here.
This messege of longing is one that gets repeated a lot for it is something that many people have and do struggle with. I have even read a couple sonnets by Shakespeare that contain this messege. However, despite repitition the messege does not lose quality or importance for it is one that expresses deep personal feelings that great art and poetry are made from.