Complex sentence: When they finally discover the answer, Duke goes to Mary to celebrate, but she rejects him: since he wouldn't let her into his life when he was at his lowest, she feels that they cannot have a real relationship.
Split into simpler sentences: When they finally discover the answer, Duke goes to Mary to celebrate, but she rejects him. Since he wouldn't let her into his life when he was at his lowest, she feels that they cannot have a real relationship.