There is no "right" way to tell her. Just sit her down, explain that you are having these feelings. Above all try to end your relationship on a pleasant friendly note. If she comes away thinking "I still have a friend in him" then she will be less likely to tell people about your questioning your sexuality.
I have one question though. How would it "destroy your life" if other people knew about it?
~ The journey of a thousand miles begins with a single step ~
* Nothing in the world has ever been accomplished without passion *