How to stop being so insecure?

it is not wrong to think like this. So far she is good to you.Enjoy it. She is a human being. Changing her mind one day(you are not alone,there are many couples who have changed their mind about partners.) If you love some one you want her/him to be happy. If there is some one who makes her happy..you should be happy for her. We all die some day...enjoy the best moment and be thankful. Good luck...my dear