"No one knows my heart" expresses that no one really understands his pain and inner complexity, which may imply that the author feels misunderstood or lonely. "I fell in love with the star by mistake" means that this kind of pain and the pursuit of self-identity lead to the author's love or sustenance for the star.
Generally speaking, this poem expresses a feeling of loneliness and loss, as well as the desire for self-identity and the pursuit of inner truth.