Heart of stone?

In the book This Lullaby by: Sara dessen Remy finds herself caught in a world that she isnt very familair with. In the book when Remy agarees to go on a date with Dexter she says to her mom "I have been a haert of stone for so many years, Its like her broke me." and when she sadi that it got me thinking, what exactly is a heart of stone and how can one break it? When i thought about it some more I decided that a heart of stone isn't something that you can just aquire you have to be through good and bad times to get it. I can relate to Remy and try to find out what a heart of stone is at this point in the story because I too have been through nsome rough times and I think we all have so does that technaicly mean that we all have a heart of stone? That also go me thinking about well maybe we all have a heart of stone but we are all stronger about certian things for example one p[erson might be alot stronger mentally when it comes to math but another person might be stronge in the common sense deparment. I geuss my point  is really everybody has a heart of stone but we all have different "cracks" in them, so we are all strong in some way.  I still haven't figured out how somebody could break our sones of heart because if we built our stone hearts from past experiences wouldn't we know not to let anybody attempt to break it? That has really been all my thinking for this book so far. It really is quite the fantastic book. Sara dessen is fantastic author.