I posted about this a little while back..I think it will help you...
I'm young, but I was having problems with jealousy mostly with my crazy ex gf and a little with my current gf, when I knew deep down there was no realisatic foundation to back any of my fantasies up.
I talked to my bro about it. Hes older, and has a shyt load of experience with girls. He told me the key to handling jealousy is a little word called perspective.
The trouble with trying to convince yourself that you're the "shyt" and that shed be crazy to leave you or cheat on you is that when it happens, that very fantasy about yourself that you used as your sole defense crumbles.
"Man, I'm the shyt, but she cheated in me with an obese gorilla!"
That won't help you. Rather, what helps is using your rationality as a man. You're obviously with this girl because deep down you trust her, other wise you wouldnt be with her, right? (I hope so)
Having a good healthy perspective of the world lets real world reality set in. It begs the question "I'm with her and I trust her, but WHAT WOULD ACTUALLY HAPPEN IF SHE WERE TO CHEAT ON ME?"
Are you going to marry this chick? Probably not. Chances are you like her, you might even like her a lot (or even love her), but what would your life be like if all of a sudden if she wasn't around. Would you be fine?
I know I would be. Thats how I'm getting over the jealousy thing. I let my girls do their own thing (as long as its not disrespectful to me) because at the end of the day I know I'm going to be fine with myself, all by myself, if they do decide to stray. I have my own shyt going on for me in this world. i learned that you can never really control anybody else's beahviour. Like many ppl have mentioned before, she will cheat on you at some point if that is her desire. The KEY is how you respond to this reality.
Perspective gives you the ultimate confidence. "Realize" that you don't "need" any chick (because in all reality noone really does, it's just that thousands of guys on this site are unaware of this at the moment) and jealousy won't faze you a bit.
This does not mean that you should be oblivious to your girl's actions. Be aware, but don't let all the small stuff get to you.
Why?
Because you're a man with perspective.