mermaid

in the version that i know,
she tries to become part of his life but can't,
it's too painful,
she has to give up too much of herself,
she can't breathe.
in the end, she can't be with the one she loves
because she can't exist in the "real" world,
she can only live in hers.

strange how things work out,
though i'm sure this is not exactly the reference you had in mind.
it's not the only tale from my childhood that has become part of my reality lately.