Indeed this scene is so painful to contemplate.
'...you'd kill me for needin somethin that I hardly never get.'
I don't think it was just the sexual aspect that Jack had in mind.
The dozy embrace flashback reinforces this.
Jack need someone to love him the same way he would: body and soul.
Ennis was that person, but, as I said before, time and recriminations all got in the way.
Jack wore that short leash for so long because he could not get away from that love. 'I wish I knew how to quit you': Jack didn't know how to quit Ennis. I could try, but he could not.
On part of this I agree. Jack needed far more from Ennis than just sex, and thus his inability to make it on the 2-3 high altitude f**ks...
The short leash was emotional/spiritual as much as physical. He repeatedly put himself out there for Ennis emotionally. Particular examples:
The initial bluff: I swear I didn't know we were going to get into this again" followed immediately by the admission that Yes I did, couldn't get here fast enough, and the pointed question: how about you? And Ennis answers with: "uh...I don't know..." Jack insn't asking about sex here, they've already taken care of that.
The whole scene after Ennis's divorce, and Ennis doing his best clam impersonation. In contast with some others, I am absolutely convinced this wasn't just about sex either, though Jack did go seek it out as a palliative for his heartbreak.
And that agonized admission: "Sometimes I miss you so much, I can hardly stand it." Answered by Ennis with silence. Again, presumably, in this scene, the physical needs have already been addressed. ( And don't misunderstand me, I'm not trying to dump on our Ennis here; he expresses his love by doing and sacrificing, but Jack seems to need some verbal confirmation, that Ennis is repeatedly unable to give him.
For these reasons among others, I still have to believe that Jack was saying "Farewell, my dearest friend and my fondest hope" in his mind and heart as he watched Ennis drive away after their confrontation. And those of you who are uncomfortable with this should note that the film references Jack taking up with Randall not once, not twice, but three times, unless you believe that he really was fooling around with the ranch foreman's wife, not the ranch foreman himself, which I believe is a case of gender switching that Jack uses in deference to Ennis's homophobia. No way is Randall
ever going to be able to replace Ennis in Jack's heart, (a truth of which I am sure Jack is painfully aware) but after 20 years of the short leash, and Ennis's declaration of not being able to stand this anymore, it seems clear that Jack had no choice but to pursue what other options were available to him.