Complex sentence: Holland realizes that his career in music is likely over, thinking that his former students have mostly forgotten him and is dejected at his failure to ever have his composition, which he views as his life's work, performed.
Split into simpler sentences: Holland realizes that his career in music is likely over. He believes that his former students have mostly forgotten him and is dejected at his failure to ever have his composition, which he views as his life's work, performed.