Sorry if this has already been mentioned (I hate duplicating ideas) but I haven't scrolled back and read all of the posts in this thread. We say "I swear" here in the Midwest all the time. Actually I was very surpirised at all the confusion over this. It's a very familiar term with Midwesterners, and it can mean a whole host of things, depending on the situation.
"I swear I wish I remembered to bring my charge card with me" (Self admonition for forgetting to bring the charge card)
"I swear I can't remember what I had for dinner yesterday" (Reassuring the listener that the speaker, after much thought, honestly cannot remember)
"I swear I didn't do that" (sincere promise)
"I swear if you ever do that again..." (a threat to the listener)
"Jack.... I swear" (In this case, I think Ennis is musing on all the chances he had with Jack and missed out on. Ennis is saying "Jack, I swear if I had the chance to do this all over again, things would turn out different") or {"I can't believe I let this all slip away").
Now, I SWEAR I will go back and read all these posts, and find out this has already been mentioned a dozen times!
Sorry about that folks!