Complex sentence: It was at this time, upon realizing how despondent he would feel to lose her, that Lewis recognized that he had fallen in love with her, writing to a friend new beauty and new tragedy have entered my life.
Split into simpler sentences: It was at this time that Lewis recognized that he had fallen in love with her, realizing how despondent he would feel to lose her. He wrote to a friend: new beauty and new tragedy have entered my life.