Dear Cj. Great report.
Years ago when I was taking martial arts, the teacher said that he really admired me for sticking with it despite a total lack of talent! It was a wierd sort of compliment, but I think later when I got to teach a bit I found I really could understand what the beginers were struggling with. I'd had to analyse what I was doing and not just have it come naturally.
That Las Vegas event was a very tough course and you were tired going in to it. The next couple of tri's will be better, and you'll know you can pull through. I agree with previous posters that it is awesome to help the other guy finish. Don't be too hard on yourself--think about even begin able to complete this distance compared to where you describe yourself a few short months ago.
Start planning for next year. I used the Triathlete's Bible and the thing that helped me the most was to really schedule some rest time. I am not nearly as wiped out all the time nor as grumpy (though don't ask any family members!)
Look for me at the next triathlon--I'll be the last guy out of the water!
a.c.