Nauseous from past experiences?

So last year when riding back home on a bus from a field trip I got sick. It was because I had eaten a ton of junk food and it was like a three hour drive home. Ever since I've been unable to eat a lot when I know I'm going to be away from home for a while or when there is not a bathroom in the near vicinity. It is really annoying and I'm just terrified of getting sick in public. The panic of the possibility of vomiting just makes my stomach even more upset. And I have a baseball road trip on Friday and I'm really nervous that this will affect me. How can I calm myself down and get rid of this ridiculous worry?