Also, the very last part of the story says (paraphrasing, don't have it right in front of me) "There was some open space between what he knew and what he tried to believe, and if you can't fix it, you got a stand it."
I took this to mean that he KNEW Jack was murdered, but he wanted to BELIEVE it was an accident. So in some part his grief was due to having to "stand" knowing Jack was murdered, probably while he (Jack) was searching for a temporary substitute for Ennis. That would tie into his "Jack, I swear..." which I believe meant many, many things--all his regrets, all the unexpressed love, the repression, the rejections of Jack--clearly more than he was able to verbalize.