My question is about the differences between "heart" and "mind". I read many examples with these two words, yet I can't tell the differences between them when I see "greed in the heart" or "greed in the mind", and “you are always in my heart” or "you are always on my mind".
Heart is generally emotional, sentimental and viewed as authentic.
Mind is contemplative, visionary and intellectual.
Having someone's "best interests in mind" makes it sound as though the actor is going to be thinking of them in a concerned but detached manner. That statement makes it seem as though the concerns of the other person are an abstract problem to solve.
Having someone's "best interests at heart" implies that there is a deeper empathic relationship, and a more personal connection.