You train your mind to think differently. Usually guys that are jealous and fearful of their girl being attracted to someone else also get really turned on by the idea. It is like a conflict. You want her all to yourself, then you want her to have fun with others. You want her to look hot for you and to show off to others, but then others will want her and who will steal her from you. The thing is, you cannot control her when you are away from her, so enjoy imagining what she might do. The thing to know is that the conflict is normal. The desire to show her off is normal. The desire to have her all to yourself is normal. The desire for her to have others is normal. As you sort it out, you will come to respect her more and you will find it possible to let her go with her friends and trust her to be your girl when she comes back. The more fun you are to be with the more she will value your company.
You can't, as well I would have to agree with everything that "DiaTheFlu" said. Also remember this currently the female to male ratio on this planet is almost 3 -1 meaning there are almost 3x's as many women on this planet currently than there are men. So even if this chick doesn't work out, there are at least 2 more in the world for you.
Can't stop the feeling, you can stop acting on it.