She CAN do whatever she wants, you're her boyfriend, not her father, or her "owner"
Let me tell you something...
Sometimes I feel like this, that if my girlfriend were to meet up alone with a guy IN A BAR, I would indeed feel bad, and she would know its not right to do so. She probably wouldn't do it, but it would make me feel bad.
Unless you have yourself gathered up, trust in yourself and trust in her. You can even say affirmations to yourself.
Shitlike
"Im the worlds greatest lover"
"Im a beast"
"Anywhere I go girls want to fuck me"
"All eyes are on me"

You know this kind of stuff it does help! Say good stuff to yourself, and don't think of the bad stuff. Your mind will play tricks on you, the mind is very powerful, if you think that she is cheating on you, it will fuck you up big time! There was a time at the beginning of my current relationship that I was very insecure, and I even had a dream of her kissing a guy infront of me, needless to say, when I saw her again, all insecurities were gone, and I got myself together. I said to myself good stuff, drive out all the negative thoughts...
Its all in your head, its all in your head.

He has the right idea...you have to come to terms that with the distance in your relationship both of you are inherently going to miss out on your most valued relationship and for both of you to deny the social being inside is detremental to your bond. let her enjoy herself and you sir need to enjoy yourself, she has a life, get yourself one, be committed but not isolated. The more she sees that she is not your entire focus she will realize one of two things...1.finds out that these guys have nothing compared to you and values you even more...or 2. goes out, meets someone and finds him more valuable than you and will end the relationship......but with your apparent insecurities most likely be the latter...work on your inner game and dont make her your focus...build your life and LET her be part of it dont build it for her...