Complex sentence: In his autobiography, Diego Rivera would write that the day Kahlo died was the most tragic day of his life, adding that, too late, he had realized that the most wonderful part of his life had been his love for her.
Split into simpler sentences: In his autobiography, Diego Rivera would write that the day Kahlo died was the most tragic day of his life. He had realized too late that the most wonderful part of his life had been his love for her.