I have read carefully all of the comments everyone has made in the development of this story, and no, I didn't think that there would be another episode of Ennis's mourning here, but it occurs to me now something I have observed in myself and also in more than one widower friend I have known who lives long term with the loss of a loved one. Ennis got through his legbone with mild to moderate frustration, adapted to it and became relatively stoic and accepting. But for Ennis, a period of sexual abstinence and even solitude is nothing new - he practiced that for years and years! At the beginning of his legbone he was frustrated and unhappy, and it was during that time, when his sexual relationship with Ellery was threatened, that they made their vows. And then he entered an old yet familiar condition of abstinence and solitude - or at least, partial solitude. I have heard it said again and again that stoic people can get through hard times, because they are used to them, and Ennis's life was one hard time after another.
Then - he got better, and he has positive things to look forward to, new horse-training opportunities and security at his job, a joyful sexual reunion, and that ring on his finger - and everyone wants to celebrate. But Ennis doesn't know anything about celebrations or happiness. One of the earliest lines in Brokeback Mountain is "They were injured to the stoic life." He is not inured to sudden joy and happiness, and so he falls back into his grief. It is when things go RIGHT that he falls apart, because that is when he has the space in which to grieve. Nothing threatens, no unhappiness looms for him to grapple with and endure. In this sense - hardships and problems seem right, and it is bliss and happiness that seem wrong, and that is when Ennis suffers most.