Boku no Shotaiken

Despondent that the girl he loves does not return his affections, Eitaro attempts to commit suicide. But his near dead-body is discovered by a scientist and his mind is transferred to the body of a young girl. As a girl, Eitaro soon lears that the girl of his dreams did in fact love him, so they try to make the relationship work despite the fact that Eitaro is in a girl's body.
Source: ANN