Ennis was not exactly the wordy type, right? So he's kind of mumbly and he does say "jack, I don't know what to say." You can read the misery on Ennis' face. It IS who Ennis is! And he is very very clear that he is NOT going to be with Jack in the way Jack wants him to. I don't see Ennis handling it any other way, and that's what his stance always was. What a heartbreaker.
Heartbreaker? He was brought up scared of the consequences of anyone ever knowing he's gay, so to have Jack showing up and Jack talking about them living together makes Ennis afraid. His fear overpowers him so much that he can't even embrace the man he loves and instead pushes him away, hides him in a corner, keeps him out of sight of others, and tries not to think of how it could be, always, of how it could have been.
The fear just kills him, you can see the pain in Ennis' eyes...Jack is heartbroken because he doesn't understand that fear,he's willing to stand up to anything if it'll get him what he wants, but Ennis just doesn't have that, can't think like that. Thus, the argument, and Jack saying "I wish I knew how to quit you!"
It's the whole "goddamn bitch of an unsatisfactory situation," as Jack says, that is utterly heartbreaking. Not, in my opinion, Ennis in particular.