In what I wrote originally about this, (posting 985 above) I had in mind the ultimate conclusions drawn from surveying ones life, rather than those situations we face in the midst of life that can still be "fixed," gone on from, triumphed over. Proceeding from the Serenity Prayer, perhaps the wisdom involved is knowing when to say "enough is enough." I'm not talking about suicide, just the recognition that it is time to "stand it." But let's take it back to BBM, and compare Ennis to George, the professor, in Single Man (postings 190--195 at that site).
Ennis and Jack were 19 when they began, 39 when Jack died. Except for the first summer and their deepening love for each other, they had to "make it on a couple of high-altitude fucks once or twice a year." When Jack died they were only 39, but Ennis faced a meaningless future, his life was going nowhere, and with all his hangups his prospects of finding a new Jack were remote indeed. Ennis did not lack "wisdom" to "stand it:" Given his whole being, he could do no other, except perhaps to sink into absolute despair. He really was doing the best he could. (And I will not comment on another man's courage.)
The situation in Single Man is similar in many ways, but different in crucial ones. George and Jim were together, really together, for 16 years. When Jim died, George had been grieving for a year before the events of the last day shown in the film. I was irritated that after a year of grief George was not moving on. I was taken to task by Southendmd, milomorris, and Jeff Wrangler for this shortsighted view, and I backtracked. I agree that in putting away the gun George was showing that he was ready to "change it," to move on. I still can't forgive him for turning away from those 3 gorgeous men, but he did show signs that, had he lived, there would be more gorgeous men in his future. George was basically unlike Ennis and he was willing to go on. (That he was not able to because of the heart attack is a subject for another time.)
So Ennis was doomed to "stand it," while George was still in a position to "fix it." Add more years to their ages and chances of "fixing it" grow increasingly dim. At the risk of sounding simplistic, I think the message here is: if you can, get it while you can, before it is too late. "Never enough time, never enough time."