I don't think Jack quitted Ennis. Jack knew Ennis will never merge their lives together. He decided to get "outside interests" to sustain him for the periods inbetween seeing Ennis. Jack saying "I wish I know how to quit you" is actually an acknowledgement that Ennis will always be the central figure of his life, even if this hurt him so much.
If Jack hadn't die, I'm sure their relationship would have carried on as always, even if Jack had another interest at home/on his dad's ranch.
I've always believed that Jack never could quit Ennis even if he were to try. But then all 'em years driving 14 hours each way and getting nothing more than a couple high altitude f@#$ a few times a year sure put a man's love to the test.
Problem was, that day when Jack got turned back cold after the divorce, got him started looking outside, and even since, he hid it behind his mustache. They never talked about that incident, or about anything that matters, except that one time by the river, when Jack proposed.
The problem is not so much lying deliberately, but lying implicitly, by omission, by neglect. They left things as they were, nothing changed, nothing discussed...in the end, only the inevitable change of death, finally changed everything for the one left behind.
If Jack really loved Ennis, he wouldn't cheat on him, no matter what. Ennis never cheated. If Jack really loved Ennis, he wouldn't get himself killed by no tire iron, or no accident. Ennis held on to his dreary life running around the tea pot looking for the handle, but he held on, careful even in the hard life of a ranch hand.
In his mind, heading up to Lightning Flat that sunny day, Ennis wasn't sure that Jack loved him. He only knew that he missed him, and wanted anything just to be close to him. When he got 'em two shirts in the closet, all the lies were swept aside, the lies that say it was wrong to be loved by a man, the lies that say they were nothing but fishin' buddies or f@#kin' buddies, the lies that say Jack was looking to bring another guy home.
In that one brilliant moment of discovery, Ennis saw the truth, the only truth that mattered, and embraced it with all his being.