involving the heart implies emotion, so "be true to my heart" means to follow my feelings. "be true to myself" implies consideration of character, so if you are true to yourself you follow what you think is right, moral, and proper.

Being true to yourself goes toward who you are. Being true to your heart goes toward your emotions and feelings. Follow your heart goes toward doing what your heart wants. Basically they all kind of mean the same thing but it's a little different. That is because if your being true to yourself then you are following your heart and if your following your heart then your being true to your heart. Sorry if this confused you.