In my experience, jealousy comes from insecurity, or a lack of trust, or a combination of those. You need to decide why you feel you are not enough for her, why you think she'd be wanting someone else.

Has she earned your mistrust? Because jealousy is mistrust, and that's unkind towards her. If she has not earned your mistrust, then you need to CHOOSE to trust her. Work very hard on keeping your mind OFF of the "possibilities," use another focus. When you start to feel jealous, for example, deliberately change your mind to your favorite fantasy of you and your partner.

If it is earned mistrust, then you need to examine why you feel that she would repeate the behavior that created the mistrust.

But above all else, realize that you do not feel secure. Either in your own "charms" and ability to satisfy her, or in your relationship. THAT is the essence of what jealousy is. Knowing that, work hard to find a way to feel security with your relationship and with your self.