Great question, I don't know how many times I've seen "Jack, I swear..." appear in a discussion where it is used as the justifying conclusion to the argument - me included! Ok, so everyone is going to have their own version of this and every one of them will have credibility. Most will want it to be the start of a sentence that ends with the words that mean most to them. Here are a couple of endings along the lines of regret and revelation:
Jack, I swear... I won't forget you
Jack, I swear... I always loved you
Jack, I swear... I won't make the same mistake again
Jack, I swear... If you didn't die things would've been different
You could also argue that it was a more about the sadness or lonliness:
Jack, I swear... You really messed me up good
Jack, I swear... You didn't have to die to quit me
And then there's a slightly abstracted sense (regarding Alma Jr):
Jack, I swear... As long as he (Kurt) loves her, then it's ok
For me it is all of these because it is only at this point that Ennis is finally redeemed and accepted his love for Jack.
I'd be interested if people have some more lines or contexts that I haven't thought of yet.