Hello Loreen.

Unfortunately, Ennis's inner distractions were too hard to accept Jack's love completely,and Jack DID UNDERSTAND THAT! But he needed more... and Randall was just a convenient opportunity. Nothing more than that!
I disagree with you here, sorry. Jack didn’t understand that. First Jack thought that Ennis couldn’t be with him because he was married and he couldn’t leave Alma and the girls. That’s why Jack was so happy when he learned that Ennis had divorced. But then he found out that Alma was not the reason that kept them apart. You can see his slow realization when he follows Ennis’ eyes when he looks at that truck on the road. Jack knew then that Ennis was not with him because of his fears, yes, but also because of his paranoia. If Jack had understood Ennis reasons to be apart, he wouldn’t have been that bitter and hurt for so many years. He sure loved Ennis beyond limits but he didn’t get why they couldn’t live together. I mean, in the last confrontation when Jack said to Ennis that they could have had a good life together, he doesn’t said “but you couldn’t Ennis” but “but you did NOT want, Ennis”. Jack thought that Ennis simply didn’t want to live with him, you know. Because Jack was also scared, but when you love someone you fight and take changes and risks, and that’s why Jack didn’t get why Ennis didn’t act like him.
Another thing: if Jack was really giving up on Ennis, would he be so irritated because of so late meeting (in November?)...No! He would've say: OK; Ennis, you just keep on working, I found myself a man who always have enough time, with available cabin, and a good will to take me to the heaven!
Well, baby, if you see the film again you’ll hear Jack saying before the last confrontation that he was gonna go and see his parents after they parted. And then you can see that OMT said that Jack told him about that ranch foreman the last spring. Jack used to see his folks after every fishing trip, and the very Jack said to Ennis that he was gonna go to LF now. So Jack told his father his plans about the foreman after the confrontation scene, when he left the mountain and went to LF.
I’ve never thought not even for a second that Jack gave up on Ennis, and I’m sure he’d have been on the mountain coming November. But it’s sure that Jack was terrible hurting that spring, and maybe, like Thomas said once, he thought that it was the best for Ennis if he left Ennis alone, because Jack saw that Ennis was suffering a lot having that relationship with him, and that he renounce lots of things to be with Jack. So sometimes the greatest prove of love is leave your lover be, quit him, and sacrifice yourself in order to give him peace. Maybe Jack wanted to give up on Ennis not for himself but for Ennis, you know, to stop hurting him, because Ennis said that he was like that because of Jack (I'm nothin. I'm nowhere). I don’t know.
But Jack didn't say that! He embraced Ennis, and hold him tight,, letting him know that he's there for him always. If Jack hadn't gone, he'd still be happy to drive 14 hours just to make love to his Brokeback mountain friend and lover!
Like I’ve said, yeah, he’d have been there, but Jack was NOT happy anymore driving 14 hours to see his lover. That’s why Jack was so bitter and angry and desperate. That was not enough for him. Those fishing trips were more and more infrequent, and briefer, and Jack was so hungry for more. He needed to live with Ennis the whole damn time, every day, and not only a week in the middle of nowhere, hidden like if they were criminals. Jack said yes to Ennis terms when they were young because in that moment it was all whet he could do to not loose Ennis. Little is better than nothing. But now it hurt so much. Jack wants more, still is dreaming, but he can see that it’ll always like that, meeting on a mountain, never live together, and Jack is bitter and disappointed. He still keeps meeting with Ennis, because he can’t help it, he loves him so so so much, but happy to drive 14 hours to spend only seven days with Ennis? Mmmm..if that was like that Jack had not been that angry and serious and sad. Jesus, you can see his face that last night they spent together. Where was that chatty, happy, sweet smiling man? He was just a broken ghost of what he was. He was bleeding inside. All those years of !@#$ fishing trips were killing him slowly. That last trip he finally understood that Ennis will never live with him...and that shattered him, and sure he was so blind with pain that he started to talk about Randall, because he only wanted a man by his side, someone to love and be loved by. Jack was only trying to find a substitute for Ennis, to try to forget him, because maybe he thought that he was wasting his life, chasing dreams to never come true. He maybe thought that he deserved more than hidding on a mountain. He was not happy anymore with those fishing trips, and besides they were more infrequent because of Ennis jobs, so maybe for Jack it was another sign saying that Ennis didn’t want him like before.
Some brokies here said that even if Jack had given up on Ennis, that doesn’t mean that their love had died. Jack couldn’t stop love Ennis, and no matter if he was gonna live with Randall or not, Ennis got his whole heart, Ennis will always and forever be the love of his life.
Uh! 
I found so many of you take Randall as someone important in Jack's life.... Well I don't! It's just that I follow both SS and the movie, and there's no room for no one else there. Jack had been with many other men, and if he's beaten to death (Most probably!), that was not because someone found out about Ennis, but rather bacause of promiskuitet behavior, which could've attract the attention of murderers!
But Randall itself... for me he's only an episode, someone near, available, handsome and willing to do it!
And I could not possibly imagine that dandy to go to live in LF??? I truly don't think so! 
Well, I’m sure Jack didn’t love Randall, but Randall was not only a f*ck buddy. I mean, they spent almost 5 years together (you can see that in the Screenplay), so they were friends for sure, good friends.
All of this is only my humble opinion, and please notice that I’d written lots of “maybe”, because I don’t know, we don’t know for sure.
Sorry for the big OT.
