The story Ethan Frome is rich in allegories which point out the disordered and chaotic life of all of the main characters, especially Ethan. Yet, to find the best quote (and there is more than one) one has to analyze Ethan's personality in order to decide what would be the best fit.
Ethan is a man whose life has seen nothing but obstacles. He was born in a poor household that became more poor with the passing of time. His father became mentally unstable and died. Ethan's mother also became mentally ill until her death.
Regardless of the meager conditions in which he was raised, Ethan demonstrated an interest in becoming educated. However, he missed his chance at getting an education. Perhaps all his intellectuality hardened up with the constant rough patches that he had to endure.
His wife Zeena was the primary care taker of Ethan's parents when they became ill. Ethan's proposal of marriage was made more out of gratitude than out of love. We know that Ethan was not happy with Zeena, that he had fallen in love with Mattie, and that they are under consistent pressure from Zeena's presence. In an act of desperation they made a suicide pact that failed, leaving both Mattie and Ethan crippled.
Hence, Ethan completed a sad full-circle with life: He was born and raised under a chaotic household, was married and lived in a depressing marriage, and will now end his days in chaos since Zeena chose to take Mattie in to be her caretaker.
As far as a quote to describe all this, we can probably find it in the introduction of the novel, when Harmon tells the main character and narrator about the life of Ethan Frome. His quotes summarize Ethan's life:
When a man's been setting round like a hulk for twenty years or more, seeing things that want doing, it eats inter him, and he loses his grit.
Sickness and trouble: that's what Ethan's had his plate full up with, ever since the very first helping.