Be careful its my heart

Sweetheart of mine, I've sent you a valentine.
Sweetheart of mine, it's more than a valentine.
Be careful, it's my heart.
It's not my watch you're holding, it's my heart.
It's not the note I sent you that you quickly burned.
It's not the 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.