Have you surrounded your heart with concrete walls?

A woman I have begun to fall in love with has very thick walls built up. I haven't quite figured out why, but she is good at keeping a calculated distance. It is terrifying. I'm scared that she will never love me, or at some point she will get too scared and run off.

Has anyone ever taken the time to break down your walls? How did they go about it? What would you suggest for someone in my position?

She has my heart. All I want is for her to let me in. Just a little.