A woman accused of fatally stabbing a 7-year-old girl was repeatedly failed by the state’s mental health systems, according to her public defender, who argued the weekend’s tragedy “could have been avoided.” Shavon Gayden, 48, was set to appear at an initial court hearing Wednesday, but it was rescheduled for a second time because she remained hospitalized. Gayden had a history of committing seemingly random acts of violence — once attacking a C…