To cry your heart
To weep a lot and feel really badly about something
To be cold-hearted
Lacking in sympathy
To have a change of heart
To change your mind
To take something to heart
To have your feelings hurt by something someone said or did
To have a big heart
To be giving, caring
To cross your heart and hope to die
To promise (que me caiga muerto/a/que me parta un rayo)
From the bottom of your heart
To really mean something
To have your heart in your mouth
To be scared or nervous
To have your heart set on something
To really want something
To have a heart of gold
To care about other people