Hi, I'd like to help you with your question.
Jealousy is a painful and frustrating feeling that is hard to control. It often is used as a way of controlling another person. It is also destructive, as you are experiencing with your relationship.
The key to resolving your problem is to realize that jealousy is not about keeping track of what your girlfriend is doing and if she will cheat on you. It is about how insecure you feel as a person. Being insecure, you naturally assume that your girlfriend will not love you and she will hurt you in some way. This leads you to try to control her, to keep track of her and to overly express your feelings to her. It also can escalate to the point that you blame her for making you feel this way.
One of the first steps in resolving your feelings is to recognize your feelings of insecurity. Then takes steps to address it. Work on learning more about how to pay attention to who you really are and working to feel more confident. Also, realizing that self esteem is about feeling you are worthy and not allowing your worth to be determined by others is a big step in recovery.
Talk to your girlfriend about working on this issue. Tell her that you are actively seeking ways to improve how you feel and stop letting this interfere with your relationship. Ask her to help you. Here are some resources you both can use:
Romantic Jealousy: Causes, Symptoms, Cures by Ayala Malakh-Pines
Life Without Jealousy: A Practical Guide (10-Step Empowerment Series) by Lynda Bevan
You can find these books on Amazon.com or your local library may have them for you.
Once your girlfriend realizes that you are taking steps to improve your problem, she will feel better about your relationship.
I hope this has helped you,