I think that Ennis's "once burned" response was do to the fact that he knew he should never gotten married to a woman in the first place. And, I also thought that Ennis felt pressured, or at least thought he was being pressured, into getting married.
I know I am repeating myself here or what I have posted in another subject thread; but, if Ennis had realized immediately why he thought he felt like someone was pulling his guts out and he thought he had to puke, he would have made an effort to get in touch with or follow Jack up to Lightning Flat. While he was walking in the movie, Ennis was driving in the original story.
While I did not feel like some was pulling on my guts during a separation period for financial reasons, with Ed staying back in California, I had a very empty feeling after I had gotten a motel room the first night on the road. I had left that morning before Ed got home from work (he had agreed with what I thought was best); but, when he got home from work that morning, he told me later, "When I got in the apartment and you weren't there, I wished I had never let you go."
I also think that Alma Beers might have been somewhat immature when they got married, although she was old enough. I have heard women lacking in maturity speak to their husbands or live-in boyfriends like Alma often did in the story and the movie. Some of them even had separation anxiety problems.