I Give My First Love to You (2009)

As a child Takuma is diagnosed with a heart condition that requires care from a cardiologist. He soon becomes friends with his doctors young daughter Mayu while being treated at the hospital. One day Mayu overhears her father saying that Takumas condition is serious enough that he most likely wont live beyond the age of 20. Takuma isn't aware of this himself, however, so at 8-years-old he makes a promise to marry Mayu when they turn 20. Later in life, Takuma becomes aware that he wont be able to keep this promise and he begins distancing himself from Mayu in hopes of protecting her.

Source: TMDb