I think the "I swear" line means, "I swear, if I had known things would have turned out this way, I would have taken a chance, found the way for the two of us to have a life together." (Or perhaps that's my own wishful thinking.)
That final line is all about regret and the sting of things that cannot be undone. I think Ennis realizes too late how deeply he truly loved Jack. The "I swear" broken off, is also symbolic of their uncompleted, "broken off" love affair. Someone used the word "thwarted" in a recent post, referring to the John Twist, the "stud duck". That word applies to Jack's life, Ennis's life, Ennis parents lives, Jack's parent's live ( and the lives of countless others who are trapped into just existing with no real joy.)
In those two words, we are made to feel Ennis' regret, frustration and grief. My mind keeps going back to that question- "I swear"? It's a mystery that won't ever be resolved. Proulx is a brilliant writer. She shows Ennis to be taciturn and enigmatic even until the end. It makes you want to reach out and try to get inside his head, break those barriers down, comfort him. Probably why Jack loved him so much. Ennis was so obviously wounded, Jack wanted to "fix" him, wanted to give him happiness, a good life after the bad hand he was dealt as a kid.
God, the way Ennis inhales those shirts really did me in. I sobbed in the car all the way home!