Most Helpful Girl
Explain to her that your jealousy has nothing to do with any wrong in her, but more your own insecurities. Let her know that you will try to work on it and become more confident in her remaining faithful. You can't stress enough that you do trust her, and it's just something you'll have to work on in yourself.
I totally know how you feel as I'm in the same boat as you. For one reason or another I also get pretty jealous which afterwards I see is often irrational and unwarranted. I know my boyfriend would never cheat on me but still... I'm going to try to work on it despite him being willing to make certain sacrifices and not hang out with certain people. I realize that I'm only hurting myself by getting so stressed out, and by working on that issue the relationship in the long-run will certainly benefit.