I definitely understand anticipatory stress. With all the books I've read on managing stress, with all my yoga and meditation, I was still stressed before going in to New York City to accept my two book awards. I knew logically that it was not helpful to be stressed - and yet I could not quite override my body's natural reactions.
Still, there are ways to ease them.
* eat healthy foods / drink water / avoid caffeine. The more healthy nutrients your body has to work with, the more even keel it will be. So that gives you a solid foundation to begin with.
* get ample sleep. I know this is hard with stress! Do your best. Darken your room. Block all little lights. Make it cool. Make it quiet or use a noise generating machine of some sort. Journal before you sleep to help reduce those circling thoughts.
* Make a list to prepare. The more you feel "in control" of what you can control, the more relaxed you will be.
* Accept what you can't control. There are all sorts of things in life we can't control. It doesn't make sense to worry about them. Just accept they will be what they are, and that life will go on. Yes it could have been rainy on the trip. And in fact it was! That was all right, we drove in the rain. They forgot to call my name for my Gold award. That's OK, Bob went up and reminded them. So solutions are found. Life goes on.
* Long, deep breaths. We tend to shallow breathe when we're stressed. That then creates a feedback loop in our brain telling our body that we should be worried. Instead, take long, deep breaths. That creates a feedback loop that everything is OK.