Also built into the question, I think, is Ennis' acknowledgment that he didn't really love her mother, and that he hurt her profoundly by living a lie. Not only is he acknowledging, of course, that he loves Jack, but that he doesn't want Alma, Jr. to go through the pain her mother did by marrying someone whose love for her isn't true.
I agree - it's the defining moment of the film. And like I say, it really does take my breath away (at least I always find myself holding it, then finally exhaling) every time I watch it.