I also think, like others have said before me, that "the rest" is not just one spesific thing but a number of things. The way I understand it, Jack is saying "I hope you know that" - that all we have is Brokeback mountain and we have nothing beyond that, because you wouldn't commit to our love and you wouldn't come and live with me, so I hope you know that this situation is your fault, "if you don't never know the rest" - if you never know how much I love you, how much I need you, if you never know how much I miss you and how much it hurts, and how bad it gets, to the point that I seek relief in the arms of other men just because I can't be with you... There are a lot of things that Ennis doesn't know or realise until it's too late.