I'm confused and sad at the same time lol.
I'm not sure if I got it right but here's my interpretation.
So... both the girl and guy didn't know what the other's wish was.
Girl assumed she would die from her illness so she wished that "guy wouldn't forget about her". Guy only knew that girl didn't wish for health so he wished for her health in her stead.
But what followed was an irony: girl got healthy, but she's forgotten all about him, and guy is depressed and contemplating suicide (which is probably the price of his wish?). Regardless, they both had to face the price of their wishes.
Their love is a story of, for lack of better phrasing, selfish selflessness.