Complex sentence: Feelings of powerlessness on the part of survivors who tried to help the injured was a major cause of post-traumatic disorders experienced by some, who experienced so-called "survivor's guilt" or who believed that could have given more help.
Split into simpler sentences: Feelings of powerlessness on the part of survivors who tried to help the injured was a major cause of post-traumatic disorders. Some survivors experienced so-called "survivor's guilt", believing that they could have given more help.