I agree with Kyle. It's not jealousy: it's fear and insecurity.
How do you get over it? Talk to your partner. Find ways where she can make you feel unique.
What if he's better at sex? Then she's going to be getting some awesome sex.
What if they have more fun on dates? Then the two of you need to step it up and find things that you enjoy doing together so you can have fun too.
What if she decides to be with him instead of you and/or both of you? That is a risk you have to decide is worth taking (or not).
Everybody has something that they are good at. Each relationship has something(s) that is special. You need to figure out WHY your relationship is worth keeping.
Insecurity tends to directly reflect one's sense of self-worth. If you think you're awesome and know you're giving her something she needs (emotionally, physically, etc.) then you're not going to worry about losing her. A fear of abandonment is more difficult. Why do you think you have it? Have you had experiences in your past that make you worry about it? Has she shown you ANY indication that you need to be worried about it?