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.