I think Jack would have always mourned Ennis, but he would have made a new life without him, still being sociable, joining up with someone else..
I also think that Jack would have mourned for a time, experienced guilt (entirely misplaced guilt considering that he did much more to maintain the relationship than Ennis). But I also think that eventually he would have not only found a more compatible, sympathetic love-interest, but also probably leave Texas/Wyoming entirely. Randall would be left behind as well. Jack would probably move to a liberal, rural area and fall in love with a more suitable life-partner, an individual much more likely to be the 'love of his life' than Ennis ever was or could have ever been.
Annie gave us two characters who cannot stand alone.
Ennis and Jack *were* alone (ie. apart) for most of their relationship. And Ennis' profound emotional dysfunctionality and hypocrisy (he could have girlfriends, Jack could have affairs with women, but Ennis went berzerk about Mexico) clouded their time together.
I think that the last big fight they had ended the relationship in Jack's mind. Eventually, Jack would come to realize that Ennis needed a good therapist more than a boyfriend.