I checked out the book Close Range from the library, and re-read the story a few times. That last line I was thinking about, and this is my take on it.
What Ennis KNEW: Jack truly loved him, and he truly loved Jack. The love they had was something most people don't find, it was something truly special. Ennis finally accepts his homosexuality, understanding that he was in love with a man, and there was nothing wrong with that. Society had made him believe that there was something wrong with him if he was "queer", hence he had spent all those years fiercely struggling with his feelings and who he was, but now he finally understood that society is wrong and there is nothing wrong with him, and what he was doing was not a sin to be punished for. He is no longer in denial of his true nature, and he accepts himself and the love for Jack as something good and pure. He is no longer afraid of it, (symbolic of this is him hanging the shirts on a wall, signifying that he is no longer "in the closet" as the saying goes, and neither is Jack. Their love is now openly acknowledged.
Also, Ennis feels immense sorrow and regret at having repeatedly denied Jack and himself happiness. He has a thousand thoughts about what if, what if. What if he had never married Alma? What if he had instead gone to Ligntning Flat that fall/winter after the 1963 summer, like Jack hinted he should, or if he had decided to get in Jack's truck that last day and ride off with him? Even after that first mistake, there was still another chance four years later. What if he had accepted Jack's proposal at their reunion getaway and gone with him then? What if in all the intermittent years he had finally accepted Jack's proposals? If he had gone to Texas with him? Or Denver? What if he hadn't turned Jack away after the divorce? Think about all the chances he had, and he messed it up time and again. He blames himself for it all, and it's a tremendous burden to carry, all that regret. He lived a life of regrets, and he frankly couldn't go on for another 20, 30, 40 years without Jack with that huge burden on his shoulders.
What Ennis TRIED TO BELIEVE: So he tried to believe that what he had done was protect them. That by denying that sweet life together he had saved Jack and himself from a death similar to Rich and Earl's. Even though, he knew Jack, most likely still had been killed in hate crime, Ennis tried to believe that what he had done had given both himself and Jack twenty years of living, whereas, they would have been killed 20 or 15 or 10 years before if he had accepted Jack's proposal and decided to live his life with him. Of course, deep down, Ennis knows he is lying to himself. That what he had done (not accepted the sweet life together) had only caused both of them a lot of pain and suffering, and it had hurt Jack even more, it had not saved their lives from a violent and heinous death, it was done out of Ennis's fears, and was not worth it. But he had to make himself believe that because the regret and sorrow was too much to live with for the next thirty years, much less the next two months. It pained him too much to realize the mistakes he'd made, and now Jack was gone and he finally realized how much he'd hurt Jack by refusing, and how miserable he'd made himself.
