Actress Shannen Doherty has revealed she's battling stage four breast cancer.The Beverly Hills, 90210 star, who was previously diagnosed with breast cancer in 2015 but entered remission for several years, shared the news Tuesday in an interview with Good Morning America. "My cancer came back," Doherty said. "...I don't think I've processed it. It's a bitter pill to swallow in a lot of ways."The 48-year-old actress told GMA she kept the news private while working on a 90210 reboot but is announcing it now because it would have come out in court documents, as she's engaged in a legal battle with State Farm. In the interview, Doherty said she's "petrified" and emotionally described how after her co-star Luke Perry's death, she wondered, "why wasn't it me?""It's so weird for me to be diagnosed and then somebody who was, you know, seemingly healthy to go first," Doherty said. "It was really shocking, and the least I could do to honor him was to do that show."