Ponyboy feels betrayed because Soda has confided in Darry first, making Ponyboy feel like he wasn't the brother Soda trusted most. This betrayal is compounded by Soda's earlier statement that "Darry's always the one who gives advice." It feels to Ponyboy like Soda has abandoned him in favor of Darry.
Despite the hurt, Ponyboy also feels sympathy for Soda. He realizes Soda is struggling, and he begins to understand the pressure Soda has been facing. He feels a sense of responsibility to help Soda, and he wants to be there for him.
Ultimately, Ponyboy's feelings are a complex mix of anger, hurt, sympathy, and responsibility. He is still grappling with his own grief and the pressures of his life, but he realizes that Soda needs him, and he wants to be the brother Soda can rely on.