Hardening of the heart comes from holding grudges, not accepting that someone had to move on, not accepting the fact that the relationship wasn't compatible, etc. It's not something I would recommend.
I agree with Dexter that you have to be careful who you trust your emotions and heart to. Just because one gets butterflies in their stomach when they see or hear from someone doesn't mean that it's "love." Just because you share many of the same things doesn't mean that you'll love that person.
It's not necessary to harden your heart. What is necessary is to be careful with it and not give it away easily.
When my husband and I separated, it hurt so bad that I made up my mind that I didn't want to love someone again because of the pain. I put up my walls to the point that no man would find me attractive. I wound up hurting myself in the process. I said I didn't want to go through all that "garbage" again... so why I did I take him back over a year ago?

Maybe it was because I've learned a few things to keep myself from hurting. He still drives me nuts, though!
Dex, you're right on! You're getting to know me pretty well.
<font color=blue>"Our doubts are traitors and make us lose the good we oft might win by fearing to attempt" --Shakespeare</font color=blue>
__________________
Psalm 119:105 Thy word is a lamp unto my feet, and a light unto my path.