My fiancée is going to Vegas for the second time, about a year from when she went last time. She went to Vegas last year and ended up exchanging numbers with a guy there and became infatuated with him. They texted for a few weeks and I found out (from a text to her friend there with her) that she had strong feelings for the guy. We had a long discussion and she realized she f'd up and made a mistake. She is going back for her sister's bachelorette party tonight and is planning on doing much of the same. (i.e. pool party, clubbing, etc).

She told me before she left that I have nothing to worry about, she promised to be a good fiancée. When she went last time, we were not engaged.

I want so badly to trust her, she has done nothing to prove otherwise since. I guess I am just having trouble getting over past issues. I guess I should just let her have fun (first instinct) and trust her, but it is so hard to do. I heard from a couple married 30+ years that is you can't trust someone on a trip like this, you should just take back to ring. I love this girl, what are some good ways to just not worry and let her have fun (within reason). Please advise.

There are no techniques. It's all in your entire mental mindset. I find yours a bit troubling, because you use words like 'let her have her fun.' She isn't your daughter or chattel. She is a free adult, whether your girlfriend, fiancée, or wife. She is going. You can stay home imagining all sorts of things, or you can accept the fact that you might wonder what is going on but that you will keep a lid on it for the sake of the relationship. Your couple friends gave you good advice.
This is a test of the emergency bridecast system! Let's hope you pass.

Take it from an old married guy, its not worth worrying about what if's especially if they haven't happened yet. Plan your own good clean adult fun within reason. If you don't trust your own fiancé, you will never trust your wife and should not get married, or be engaged.

Now you can let your fears and insecurities over past acts eat at you, and make you a snoop, which I think is likely, or act like you're cool with her having her fun. That means THINK before you ACT, or SPEAK, and stay positive. As Joy points out its all about your own mindset, and that's all you can control. So make an effort to stay cool, calm, collected and in control of yourself, if only outwardly, as you learn to deal with the insecure insides of yourself. Sometimes, no most times, it takes years of practice, and there is no instant success.

Don't ever forget its your fear you are conflicted with so never project it on others.

Has she quit talking with this guy? Have you asked her if you are invited next year? She hasn't given you any reason to not trust her as long as the guy is past tense so don't sit home letting your imagination run on overtime

Has she quit talking with this guy? Have you asked her if you are invited next year? She hasnt given you any reason to not trust her as long as the guy is past tense so dont sit home letting your imagination run on overtime

She is going to a bachelorette party in Vegas. A double whammy for fear.

LOL yeah but what can he do but sit in fear or take his own vacation :D

I think I can come up with a more fun plan than sitting in fear :D. Always have. :)