Sometimes, you have done something you regret. You want to go to the past to undo what you have done. You cannot do so as you are already your-now-self. You cannot become your-past-self anymore. You cannot send a message to your-past-self.
Sometime in the future, your-future-self would have done something he regrets. He wants to go to his past to undo what he has done. He cannot do so as he is already his-now-self. He cannot become his-past-self anymore.
However, YOU can help him. His past-self is YOU. His past-self is your now-self. You can know what he has done wrong by receiving messages from him.
Live as if you were living for the second time and had acted as wrongly the first time as you are about to act now.
— Victor E. Frankl