Performance anxiety?

Okay, so I found out about this on Wednesday, and I thought I'd have gotten to grips with the idea by now, but I'm still sort of terrified. I'm having a bad week.

My boss told me that I'll be doing one of the big, scheduled tours next Tuesday. The ones with upwards of forty people, the ones that require three guides between two sites. The ones where you have to tailor your tour to fit the timetables of the other two groups. The ones where you have to swap groups after the first round and do it again for the second.

I've never done a tour for more than about a dozen people before. I've never had to time my tour against two others happening simultaneously. I've never had to come straight off one round and head into the next one. I have never, ever done a tour for a group this large before. I've only ever done the smaller, people-in-off-the-street tours.

I am legitimately, full-on nausea-style panicking right now, and I have been for pretty much the past four days. And I will probably do it, and it will probably be fine, but right now my stomach hurts with how scared I am.

I hate crowds. I hate big groups of people. I hate how noisy the big tours always are in the small spaces. And I'm going to have to stand in the middle of it, and figure out how to make myself heard, and be the person everyone in that space is looking at. While on the clock and having to make sure I get them through in time to swap groups at the end of it.

I have no idea how I'm going to get through this. I would really, really like my stomach to stop cramping about it, though.