Well,I suppose everything has been said about this open end that implies so many interpretations almost as persons have seen the movie or read the story...No matter what words we can include after this "Jack,I swear...",IMO all of them go to the same point: Ennis discovers how he has wasted his life and Jack's,finds the irony that there's in passing all this life protecting themselves to finish like Earl and Rich by denying this "sweet life" together to end precisely like them.He realises how deep was Jack's love and his own love,in spite of himself,how many times he put a denial made by his mind instead of a "I love you"made by his heart...And decides to change,although it's too late for this time of their lives,not for the others that will come thereafter.From this point onward,think,write or say what you want;everything is valid,mainly because he's swearing to himself too.