Ennis simply can't bear the idea of a life without Jack in it - and the realization that this is a possibility brings him to his knees, both literally and figuratively.
Relating my thoughts here back to what "movie Ennis" says is harder - unless you add a couple of extra words to his:
It's coz of you Jack that I'm like this....I'm nothing....I'm nowhere....without you But maybe that's just me trying to find something positive out of what is a devastating scene to both read and watch....
NoReins, I think you've hit right to the core of Ennis's anguish.
What I see through the whole film is that the rest of Ennis's life is nothing compared to the brief times he gets to spend with Jack. The spaces in between camping trips are voids for Ennis, he exists but doesn't seem to have substance in that life. He drifts almost ghostlike through his world when Jack isn't with him. He goes through the motions, waiting out the days until he can be real again, take form again. I think his relationship with Jack became the only solid thing in his world, and his only true connection (aside perhaps his daughters) is with Jack.
The threat of losing the only thing in his life that has any real meaning to him is enough to break him that last time at the lake. Both in the film, with the words, and in the short-story without.
I can definitely hear
"without you" implied in the few sentences Ennis is able to choke out at the end.
More sadness, but more confirmation of love, too. :'( I guess that makes this bitter-sweet.