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 wrongly by receiving the messages from him.
Victor E. Frankl:
“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. ”