Ruby -
I agree. I always felt that Jack mentioning the 'ranch neighbor' was just an emotional and/or defensive cover but that his mother saw right through it.
In the lake scene, when he saw Ennis was hurt emotionally, he couldn't stand it. All the resolve he was trying to build just melted away and he went to hold & reassure Ennis yet again w/ "It's alright, it's alright."
And the "Damn you Ennis" was because he knew right then that he couldn't "quit" him.