Need help identifying Japanese movie?

I believe it's a live-action version of an anime, but I can't remember. It was made post-2000. There is a boy and girl in high school, and the girl suddenly finds out she is a cyborg/government weapon meant to save the world from invading forces. While she wants to be a normal girl and carry on with her life, she realizes she was meant to help humanity and sacrifices herself. At the end of the movie, her boyfriend finds her crystallized form in the middle of a desert.