Go, Nanci, Go! Good for you for overcoming your fears/concerns and registering. I'm starting to realize that any event like this is at least 80% attitude and determination (as long as you have basic skills) and you surely have that so I bet you'll do great!
After my mild panic attack a few weeks ago in my first tri, I got a wetsuit and registered for another. I also did a mile swim in a pool to give myself a place to go in my mind should I freak out again. So if you start to get panicky in the water, just start to visualize yourself doing the swim just like you did recently so that you have something to reference. I wish I had good advice on breathing and open water tips but I'm still learning myself.
Best of luck and let us know how it goes!