"There was some open space between what he knew and what he tried to believe..."
I just want to add that on a cognitive level, what Ennis is experiencing is called "cognitive dissonance" - the psychological tension or anxiety (or "open space") created between two incompatible thoughts or beliefs, or even between an incompatible thought and a behavior. One of the ways to reduce the tension (or "fix it") is to change your perceptions, perhaps by looking at opposing and rigidly held beliefs or thoughts in a more balanced way. Since Ennis could not resolve (or "fix") many of the polarized thoughts or beliefs that he held, he had to continue to live in a perpetual state of tension/anxiety ("stand it").
If this line specifically pertains to his thoughts about Jack's death, then his long-held, irrational belief based upon chronic anxiety/fear (i.e. 'queer' = murder) holds firm in the face of any true or contradictory evidence (i.e. real accident).
Thanks for bumping this thread. I have to agree with BBM-Cat's idea that Ennis is experiencing "cognitive dissonance". IMO, what Ennis knew is that he was queer, what he tried to believe is that he was not. Or, what he knew was that he loved a man, what he tried to believe is that he had not. The ending of the short story is bleak indeed. The film version is more hopeful in that it hints at a growing self acceptance within Ennis.
Secondarily, I think that line in the story is symbolised by the title "Brokeback Mountain". I posted this elsewhere, but my thought is that a Brokeback Mountain is also known as a Swayback mountain; two roughly twin peaks joined by a ridge giving one the impression of a swayback horse. I think what Ennis knew, and what he wanted to believe are represented by the two peaks, joined, but always to be separate.
