The first thing that came to mind were personality traits that Ethan has, but if he really wanted to, he could probably control those aspects of himself, or subvert them to stronger desires. For example, he is kind to a fault, soft-spoken, submissive and passive. Those are all personality traits that leave him a prisoner of his circumstances; because of those traits, he lets other people take the reigns of his life without a fight, passively submits to their treatment, never speaking a harsh word or exerting his own will.
However, fact that he cannot control reside outside of himself. For example, his parents getting ill and needing care in combination with his poverty--that dictated he come home from his studies and take care of them himself. That was something out of his control, and set up the chain reaction of events that led to his misery later on. Secondly, his wife is a whining, petulant and manipulative woman who thrives best when she is needed or when she is being cared for herself. He can't change that; that is who she is, and her nature dicates the state of their miserable marriage and existence when the main storyline is told. Another fact is his crippled condition after the sled crash; he can't change that condition, and it is his weakened frame that keeps him at the farm, in combination with accepting the consequences of that hasty decision to take that ride with Mattie.
So, his parent's illness, Zeena's nature, and the consequences of his sled ride with Mattie--these are all facts that he cannot control, that determine his current reality when the narrator of the story comes onto the scene. I hope that those thoughts helped; good luck!