"Your heart" in this sentence can be understood as a person's deep feelings, memories or experiences. And "can't stay in the past" means that you can't stay in what has happened and is now history. "Your mind can't stay in the past" means that individuals realize that they need to let go of some old ideas and concepts in order to better adapt to the changes and challenges brought by the future; It also means that he and she embrace new life, new opportunities and bravely meet the unknown and challenges.