young at heart

absence makes the heart grow fonder

One feels more love or affection for those one loves when parted from them.

all heart

Motivated by positive emotions, especially as productive of exceptionally generous or kind behavior.

be still my heart

An admonition to oneself to calm down when overly excited, especially in terms of romantic attraction.

break someone's heart

To cause a person to feel grief, disappointment, or sadness.

bright young thing

One who is youthful, clever, eager, and high-spirited in manner and attractive in appearance.

broken heart

A feeling of grief or loss, especially after a failed romantic relationship.

change of heart

A change of one's opinion, belief or decision.

cross my heart

A declaration that the speaker is telling the truth.