flex has good points
Listen to flex. Jealousy is very toxic. It poisons everything around it. Sometimes we (and I mean all of us) can project our thoughts, desires, feelings onto someone else. It makes sense that we do this for the simple reason that if we know we feel one way or another we have to assume (not knowing otherwise) that everyone feels the way we do.
This is for the guys. If you think about or consider a fling or one night stand, even if you just entertain the idea of it, you are going to project that onto your girlfriend and become extremely anxious when she's out among other guys. Sort of like that old saying "Those who do not trust, cannot be trusted."
The other issue is a self-esteem issue. Here's a girl trying to go about her life, having fun and enjoying life, and she has a man who has no faith in her. How do you think this makes her feel about herself? Do you think she can just slip and fall and wind up with a penis in her? What if your lack of trust makes her feel guilty for wanting to have fun? Or worse, makes her begin to believe that she actually IS untrustworthy!
You can work on your problems and get over jealousy. First you have to be a trustworthy person yourself. Once you achieve that (and believe me I'm not saying you haven't) then you'll find it much easier and simple to trust her. That way it goes from "I KNEW this would happen!" to "She did WHAT?!?!?"
Secondly, remember that women don't deal with men the way men deal with women. Women have sex drives and urges and fantasies and all those goodies like we do but they don't approach them the way we do. TALK to your girl about it. Don't complain to her. DISCUSS it like adults. If you want something long term with a girl you need to communicate often. You need to know how she's feeling and she needs to express it to you.
I'm frequently surprised when a man will say to me "I can't believe she did this! I thought we were so close!" Upon a little further questioning I find out that he works an alternate shift from her and they actually have only a very small amount of personal time together throughout the week. The time they do have is usually spent catching up on things that were let go all week. this is not true quality time. Even though the time they have is fun and good, it's just not enough. This is where a woman begins to drift away. She has needs too. Not just sexual. Before I start into a book here:
1. Be trustworthy youself
2. Don't let your own insecurities wreck her life (get help with them)
3. Remember that woman are different animals than men (with different needs, wants, etc.)
You be a-ight...
Just relax and be rational and change your thought process. It can be done. I promise.