Hanbun no Tsuki ga Noboru Sora is about a boy named Yuichi who is in the hospital due to hepatitis. Many times he would try to escape only to fail due to the nurse that would beat the shit out of him. Though, he eventaully meets Rika, a very selfish girl, and starts staying there. The 2 grow closer through the series until he finally learns why she is so selfish.
The original anime is 6 episodes long, so it should be rather easy to adapt into a movie.