Will she be faithful?

I have been dating my girlfriend for 2.5 years. I am 20, she is 21. She is going to Las Vegas with 2 of her best girlfriends this summer. I am skeptical because of the famous saying, "what happens in Vegas, stays in Vegas". I love her to death and couldn't imagine being with anybody else. We have gone far in the past 2.5 years. The reason I am posting this online is because none of my friends seem to give me the answer I am looking for. About a year ago, she cheated on me. She didn't have with the guy, but she made out with him. She was drunk. I know her and her friends are going to be getting drunk daily in Vegas. After she has cheated on me I can never trust her 100%, which is slowly killing our relationship. I know I either have to deal with it and trust her, or our relationship will continue to crumble apart. She says she is faithful, that she loves me, and that she wont cheat on me again. Besides that, I am still weary. I guess I am just looking for some insight with what I should do to control my thoughts of what "could" happen. It's tearing me apart.