Complex sentence: However, Frank is stung by Mary's remarks, because he knows they are true; he has concentrated so completely on being a "success" that everything he most valued at the beginning of his career has long been left behind.
Split into simpler sentences: However, Frank is stung by Mary's remarks, because he knows they are true. He has concentrated so completely on being a "success" that everything he most valued at the beginning of his career has long been left behind.