Complex sentence: She believes that Drew is her'One,' and is deeply upset when he falls for Sherry, and may have caused the appendicitis attack which killed her, a possibility which would haunt her afterwards.
Split into simpler sentences: She believes that Drew is her'One,' and is deeply upset when he falls for Sherry. In her jealousy, she may have caused the appendicitis attack which killed Sherry, a possibility which would haunt her afterwards.