By Heide Brandes OKLAHOMA CITY, April 19 (Reuters) - When Priscilla Salyers attends Sunday's anniversary ceremony for victims of the 1995 Oklahoma City bombing, she will be thinking how far she has come in fighting depression and survivor's guilt. She and hundreds of other survivors will bow their heads at the 20th Remembrance Ceremony at the Oklahoma City National Memorial and Museum, marking the day a cargo truck with more than two tons of explosives blew up, killing 168 people.