OK, serious question: do you know anybody who suffers from addiction? I mean, real, serious, needs help addiction? Because I do. I am one. I spend every day trying to be a better person, I work at it like you honestly would not believe. Trying to build better habits. Trying to not fall into destructive ways of thinking. Things I know are bad for me, morally wrong, and destructive both to myself and others. I've made it nearly 9 years sober. That isn't said as an achievement, or in search of praise, but merely for context. I've spent 9 years fighting.
I know people that have made it longer. 15. 20. And then a relapse. My sponsor was sober 23 YEARS before his relapse. He lived everyday like a champion, guiding people, seeming to have mastered his disease. But then one night, for reasons even he can't put into words, he failed. 23 years. Lord knows how many people he helped out of tough spots. But it happened to him. After 23 years.
So yeah, I have no problem believing that a Jaime Lannister, even one that came as far he seemed to have, could be lured back into destructive behavior. He certainly didn't intend it to happen. It's not like he did all this stuff going, "Hey, you know what would be fun? If I went back to Cersei!" If you asked him, he'd tell you that he was past it. That he understood just how messed up his relationship with her was. But when that trigger came, when he thought she would die, probably alone and afraid, every instinct he had told him to go back to her.
He doesn't do it lightly. He is full of self loathing. His speech to Brienne when he leaves is proof enough of that. He recognizes full well that he is going back to something unhealthy. But it doesn't matter. He can't help himself.
