That line *is* open to a lot of interpretations, isn't it? Once you manage to not just bawl your heart out over it, of course..... Not that it's alone in that respect.
I *think* it probabe that thematically it is intended as an obvious bookend to the reunion scene when Ennis tells Jack when they hook up again that you've got to stand what you can't fix. It's the caption line and motto for their entire hidden relationship. Now, 15 years down the lane, Ennis is saying he *can't* stand it anymore - that leaves open the possibility that he finally might be wanting to have a go at actually *fixing* it? At least, it does indicate a breaking point, the end of the continuous story arc, an impending change.
All that is film technique, though - I don't think it was what Ennis himself meant. I think his words came from a painful and only half-understood realization that the tension on him from the two lives he was living was becoming absolutely unbearable - that those two lives had pulled so far apart that he couldn't manage to maintain the compartmentalization any more - the stress had created cracks and now everything was crumbling under the tension.
The side of him that loves Jack and wants to be with him is under immense stress because the relationship is suddenly openly tenuous and under threath. A break-up can't be ruled out..... not after Jack's Mexico admission and all the bitter and disappointed rest of what Jack says leading up to the "I wish I knew how to quit you". Not after Ennis's own words in response, spoken in fear more than anger. But Jack is Ennis's whole life, his love - in direct contrast to what Ennis says out loud, Ennis would be nothing *without* Jack. The prospect of losing Jack is beyond frightening to Ennis.
At the same time, the homophobic and "pretending to be straight" side of Ennis has been driving Ennis's actions towards that loss. That side has been demanding more and more of his mental resourcesl - putting more pressure on him every day. The increasing fear that people *knows* weighs on his mind and makes him go against his own deepest wishes and desires, makes him deliberately see Jack more seldom than before. I can't find any other explanation for him sitting silently when Jack says his "sometimes I miss you so much...." line, for him keeping to his decision to cancel the August meet even after that. It would have moved a rock to cancel every other appointment and obligation! And the Ennis of the first few years relationship would have quit his job and gone to meet Jack in August. Ennis uses the child support as an excuse, but to me that's what it is - an excuse. I think in a way the homophobic part of him has subconsciously been forcing him towards a breaking point. Which finally arrives when Ennis says he can't stand this any more: He can't live both lives. He'll have to chose...... Though when he says that, I don't think he knows or sees all of that or the having to choose at all clearly yet - he's just completely emotionally worn out from it all and can't take any more and says os as he breaks down. But his realization will be gradual over the next months - I think he's well on his way to it, and to even making his choice, when he meets Cassie at the Bus Station cafe.