Be careful, it's my heart

Be careful, it's my heart
It's not my watch you're holding, it's my heart
It's not the note that I sent you that you quickly burned
It's not a book I lent you that you never returned
Remember, it's my heart
The heart with which so willingly I part
It's yours to take, to keep or break
But please, before you start
Be careful, it's my heart