Wow, August7th!

What a great thread! Thank you for asking this question.
I've never read Gone with the Wind, but I love the movie. I am glad the book explained Rhett's line because I have always felt it was too abrupt and heartless. I didn't think it fitted with Rhett's love for Scarlett. Now, with the explanation about wearing out, I can see that it makes sense.
Rhett loved Scarlett, but his love was unrequited for a long time. It was always someone else, or something else. Scarlett never devoted her love to Rhett until the very end, and by that time, he was worn out...by the lack of affection, the lies, the .... There was little upside for Rhett, and a whole lot of downside.
I think the situation is not at all the same for Ennis and Jack. Although Ennis was impossibly rigid when it comes to shacking up with Jack, sharing his dream of a cow and calf operation, there is no doubt in Jack's mind, that they share something special.
As mentioned by others, he kept the two-shirts for twenty years. He cried driving away that summer of '63. He was thrilled to be re-united with Ennis. He was surprised by the intensity of Ennis kissing him. He was happy in the motel, by the river, ..., any time he was with Ennis. It's why he could hardly standing it, missing Ennis so much. It is not unrequited love...it is mutual love, shared and felt...there's no rein on this one.
It is true, however, that by the final confrontation, Jack was sick and tired of Ennis' inability to commit. He needed more, and he was willing to negotiate with Ennis if necessary. But Ennis was unable to budge.
I am not sure if Jack's love was worn out. I think he still loved Ennis deeply, and would do anything to be with Ennis, but he also recognized, and out of love for Ennis, accepted the impossibility of their situation. Jack was willing to leave broken hearted, when Ennis said in tears, in his arms, "I can't take this no more, Jack."
The genius of Ang Lee was to juxtaposition the brokenhearted gaze of Jack watching Ennis driving off, with the loving longing gaze of the younger Jack watching Ennis riding off after the dozy embrace.
No, I don't think, in my humble opinion, that Jack's love was worn out at the final confrontation. If anything, the love between them was finally confronted as such, although still not verbalized, so that Jack knew it as he gazed brokenhearted, and Ennis was awoken to it, when Cassie told him at the diner, "Girls don't fall in love with fun."
Whew...thanks for asking the question, and getting me to revisit the intense final confrontation again with fresh new perspective.
Welcome to the forum, August7th, and I hope to hear from you again and again.
