I'd like to "resuscitate" this old thread by saying that,as a resume,this phrase "...if you don't never know the rest" is,above all,a kind of love's declaration on Jack's side.It could be-and,as a matter of fact,is too...-that Ennis didn't know about Randall or his Mexico trips; but I think that specially is some intangible things that there're in that "rest":the lots of times Jack had cried for the death of his "sweet life together" dream,the things he had left and the risks he had run for being with Ennis,the time they had lost,etc.And,above all still,for me "the rest" is "How much I've loved you and I love you still,and you had never seemed to realise it."