To the OP:
It is interesting that you are experiencing these anxious and worrisome thoughts and feelings because I have also experienced them! Let me see...what has helped me is challenging my thoughts on a regular basis. Have you challenged your thoughts? When you are anxious about your relationship ending have you asked yourself, "ok, I realize I am anxious but why? My relationship is fine and my girlfriend loves me and is here for me."
Of course, if there has been any unfaithfulness in the past then you will likely be experiencing these emotions. But, I would advise challenging your thoughts and reading a book on worry...I book I suggest is How to stop worrying and start living by dale carnegie.
Also, for me I had to realize that some of my worry was a result of wanting control over everything...my boyfriend, our future, his thoughts...etc. I have to realize there are uncontrollables in life and I must accept them if they come.
Are there trust issues in your relationship that need to be resolved?
A fantastic website I would recommend is coping.org...check it out!
Hope this helps!