Wow! I think you have just pierced my heart where this scene is concerned. As angry as Jack appeared in this scene, it was obvious that he was pouring out his heart to Ennis. Jack's words may have hurt Ennis in a way, but there had to be some part of Ennis that realized he was dealing with someone who truly loved him and had put up with a lot of heartache just to be with him. Maybe part of the reason for Ennis's emotional breakdown was that he finally admitted in his heart that this relationship was never just "a thing" where Jack was concerned.
I agree, I think that was the point where Ennis finally realised the extent of Jack's feelings, that it was all very well telling Jack "if you can't fix it, you've gotta stand it", but Jack couldn't stand it anymore, he'd been trying to stand it for 20 years and it had been tearing him apart for all that time. Ennis's body language is very telling in that scene, at first he tells Jack to "go ahead" and say what he wants to say, Ennis believes he has the moral high ground because Jack's admitted to cheating on him, and as far as Ennis is concerned, nothing Jack says can justify that. But as Jack starts to let out all his frustrations, Ennis turns his back to him, as if to block out what Jack is saying, because he realises he (Ennis) has to bear some of the responsibility for Jack's actions. Ennis says "it's because of you, Jack, that I'm like this", but I think the unspoken other side to it is that Ennis realises that it's because of him that Jack is how he is, broken, lost and lonely, and that's why he had to resort to other men. Up until that point, Ennis seems unaware (or unwilling to realise) just how deep Jack's love for him is, and it's only when Jack spells it out loud and clear that he has no choice but to recognise it. Only the strongest, deepest, most intense and unconditional kind of love could cause that level of pain and anguish, but at the same time, be strong enough to keep Jack coming back for more. And Jack would have been back in November, whatever had been said, even if Jack had maybe considered living with Randall as a last resort, he would've given Ennis an ultimatum about it first, in the hope it would give Ennis the push he needed to agree to them being together.