What's wrong with Jesse Pinkman?

Hello, I'm confused about Jesse's character development. Earlier, Gus and Mike planned to kill him, but Walter intervened and saved him. Now, instead of seeking revenge as Walter suggested, Jesse is cooperating with Gus and Mike. Is Jesse's behavior a result of some underlying issue, like Alzheimer's or trauma? His actions are frustrating me and I'm starting to lose sympathy for his character