How about I tell you I am hiding my past from a loved one.
What drives you to hide your past from someone else?
We are different people, and each has their own secrets we don't want known.
I know that you are afraid of sharing, but cloistering your past from a loved one is bad. This can lead to distrust in your relationship.
So you want me to be a vulnerable person and share my weaknesses? Not today Satan. I would rather break up with  her instead.
You must have a pretty shady past, which is all the more you should say something to her about it. It can only make things better between you if she really is the one for you and vice versa.