Juulie, it sounds like you feel very badly about the situation. I'm sure you'll work through it and be a better official for it. As I've said on this board in the past, I've officiated for almost 10 years, and it's only recently (the last 2 or 3 years) that I have felt even a LITTLE comfortable talking to and handling coaches during a game. So don't beat yourself up too much. Please.
As far as your homework, one line that was told to me was the following. If you make a call and the coach won't let go, you might say "Coach, if that's the way it happened, then I missed it. But I just didn't see that."
I have to give full disclosure and tell you that I've never actually used this with a coach, so I'm not sure how it goes over. But it seems reasonable to me and allows the coach to think that he has a valid point, but also seems to bring the conversation to a close.
Hope it helps.
Chuck