There's a discussion somewhere about bookends in the film and I think there's another example here. When Ennis collapses at the lakeside, Jack goes to him and holds him, whispering "I'm sorry, it's alright" - just like he did in the second tent scene when Ennis was vulnerable (give or take the argument over whether "I'm sorry" is ever actually said in that scene)
This is partly OOT, but yes, and I enjoyed the bookends very much. There are a few discussions in the internet regarding the classical structure and cyclic episodes in the movie. The movie in my eyes is all beautiful balance and proportion. The bookends are a manifestation of this sense of balance. It is perhaps one of the most architecturally pleasing movies I have seen with respect to this feature.
All those years earlier on Brokeback he reassured Ennis that everything would be ok, that he was there for him, perhaps already knowing that he was in love with Ennis - and despite all the accusations and bitter words, when Ennis needs him, Jack is there again reassuring Ennis that he still loves him.
In connection with the main topic and this foray into architecture, I should note that Jack's intended decision to move on and share a life with someone else adds deeply to the note of tragedy that moves us so much about this film. Your above observation is essential, I think, in appreciating how significant this was, because it is Jack -- the one who constantly reassures Ennis -- who in the end can no longer reassure himself that their love can ever be brought to the perfection that he had always dreamed.
But what makes this more poigniant is that this decision to move on in no way destroys the dream. The dream of Jack remains a dream -- unfinished and unfulfilled. It is a dream that Jack inevitably passes on to Ennis, by way of his own death. And this is probably the most heartrending bookend of them all. For the dream of love derives new life in Ennis after Jack himself had turned to ashes.
I guess what I am trying to say is that Jack's decision to 'move on' becomes the vehicle by which, in the end, Ennis comes to full realization of their love. Moving on did not mean an end to love. This is obvious to anyone who has ever gone through the death of a loved one. For Jack, it meant resignation and a certain grace (even in bitterness!) to see that their love after all went beyond physical proximity and sex. That is obvious in the final recollection of that dozy embrace. He had come to realize that their love would endure because it remains a dream that is pristine -- untouched by the viscitudes of thier pained and troubled lives.
We know that that dream of love is pristine and pure: because it lived on even after Jack's death -- in that most cherished core that is Ennis himself.
Forgive this meditation, for this is what I write. Jack will always love Ennis even if the shared life he craved for will only remain a dream. This dream of love was Jack's hope, a hope cherished without reference to mundane realities.
And by virtue of this hope, I say that Jack never gave up.