VALLEJO, Calif. — Three people gathered in a classroom on a recent rainy afternoon listened intently as Derrick Cordero urged them to turn their negative feelings around. “What I’m hearing is that you’re a self-starter,” he told one participant, who had taken up gardening but yearned for a community with which to share the hobby. Cordero, 48, is guiding the discussion at Holding Hope, a weekly therapy group for people struggling with mental health. Anyone receiving mental health services through Solano County can participate. A former member, Cordero is now the group’s volunteer peer leader. H…