Try to focus on the positives of the trip, the things you will enjoy, the things to look forward to, the reasons you agreed to go in the first place, those reasons may seem like they have disappeared but it is only FEAR that has hidden them from view.
Tell yourself you will NOT be a victim to these fears, you are capable of standing on your own two feet, etc., etc. Then write all these positives down in a notebook, laptop, smartphone, piece of paper in your purse, etc. and take them with you and keep them to hand.
I used to have a piece of paper in my wallet that had at the top "You will get through, you always do. You always have, you always will, you can handle this" and then below it was a list of reasons why I wanted to do whatever it was I wanted to do. and some positives that went with it. As soon as I started getting anxious and worked up I would recall the phrase in my mind and try to recall the positives and the reasons, etc. If I struggled to remember them because FEAR got in the way then I would dig out the piece of paper. Doing that helped quell the anxiety and got me through, it sort of counter-balanced all the negativity. Having just the little phrase "You will get through, you always do" helped enormously, and every time I got through something it re-enforced my belief in myself and the phrase - so the phrase got more and more powerful (I hope that makes sense).
Then if all else fails do as Cuchculan says have something available that will distract your worried mind & keep it occupied - Music, crosswords, books, conversation, breathing exercises, etc.
And remember, just like EVERYTHING else you got through up until today, you will get through this too, no matter what happens, you'll live to tell the tale.
Best wishes, hope it helps.