Indeed, the older Jack is not the same as the young one. His soul is now harder, his heart is less yielding.
And indeed, he was kept on a short leash as far as relationship boundaries go. As Ennis was more flexible in his earlier days, he had become more rigid as time went on. This took its toll on Jack's spirit, and he also slowly hardened to the reality that there was a line set by Ennis that he could not cross.
Well said, TPE - you do come to feel that over the course of their lives, Ennis becomes more of what he already was (cautious, repressed, taciturn) and Jack becomes less of what he was (open, easy-going, optimistic) and more of something else (tougher, determined, not so willing to compromise his desires).
The reality made Jack bitter. He had chomped on the bit long enough, and he was ready to blow off steam. After 20 years, the spring of his heart was ready to recoil. Little did he know that Ennis was a pressure valve, and he was about to open it...
Excellent description of Ennis! He's so quiet you almost forget that he's also under enormous pressure, hiding from everyone in his daily life his true feelings, until, as you say, the valve opens. The fight on the fairgrounds, the fight with the driver of the truck that nearly runs him down at Thanksgiving - small releases of the pressure, misdirected but someone alway pays when emotions are suppressed too long.
Even when Jack rages at him about the "short leash", Ennis is barely able to articulate the battle he himself has been fighting for 20 years. He collapses into sobs, a physical expression of his anguish.
This whole encounter has me holding my breath to the very end - it's pure, raw emotion.