Hi Ministering angel!
Yes, i know that it was Heath's idea. Nevertheless, i see the film as a whole, as a finished "result", no matter where the decisions came from. So, for me it is all about the change for Ennis. His realization of what he had lost and why he had
a) not lived his love out fully +
b) failed to make Jack - the person he loved most in his life - happy.
For me Ennis is way pass over that "I swear I love(d) you". Because it's not enough and too superficial. Ennis has not only to deal with Jack's loss, but he has to learn how much Jack loved him (shirts), how much Jack suffered because of being away from him ("miss you so much i can hardly stand it", Lureen saying Jack's favorite place was Brokeback and also saying that he drank a lot), how Jack's dreams were shattered because of Ennis' set up rules ("as most of Jack's ideas it never came to pass"). When you're hit with this kind of realizations it's not just enough to say "I swear I love(d) you". This is something that probably didn't need to be said at all between those two.
This is my personal perception, but if somebody who I truly, deeply loved passed away due to some stupid accident and I were to find out that this person was suffering because of me not being able to fight my own demons well enough, I would try to be a better person in his / her honor - even if it's too late now. One line that strikes me very much is Lureen saying on phone "I suppose they'd appreciate it if his wishes was carried out...'Bout the ashes, I mean..." - Why did she feel the need to specify Jack's wishes being about the ashes? What kind of other wishes could he had possibly have? Maybe some wishes which only Ennis knew of?? I think even if it was not intentionally this kind of sent a message to Ennis...
I also think it is important to notice how Ennis is checking on his mailbox after putting the numbers on it at the end. It's the same kind of checking as he did with the tent "tent don't look right". And generally, Ennis does not look crashed / devastated to me at that point. It's not that he's heavily drinking, lying around in his trailer sobbing about his lost love, right? It looks like he had come to terms with Jack's loss and - although surely suffering and grieving - it looks like he has a plan, he has some idea, a goal, he's up to something. Therefore, for me this "I swear..." somehow relates to that plan - which will probably include bringing Jack's ashes to Brokeback, but there will be definitely more to that.