I have had numerous surgeries and am always nervous until I get to the hospital. Once you are there, they hardly give you a minute to worry as there are so many forms and the wonderful lingerie to put on, lols.
If you are off the scale worrying ask for a pre med to calm you down.
If you really can not face this, you can always say ' I need to take a moment to think this through' because although its one of the safest surgeries, it a big one. Your eating will be changed forever. If you can hold your nerve in the end you will be so happy you didn't pause your journey.