the fullness of (one's) heart
An overwhelming abundance of emotion, whether positive or negative. From the fullness of my heart, I apologize to anyone who may have been hurt by my words or actions. It is out of the fullness of my heart that I address you today. Never in my wildest dreams did I imagine I'd be standing before you today as an elected official in this great state.
the fullness of your (or the) heart
great or overwhelming emotion. literary