How do the letters force Sybil to face her past?

According to the provided Recapy summary, Sybil has long coped by writing letters—many of them never sent, including one she writes to a single person over and over. When letters arrive from someone in her past, they reopen a painful period she had been keeping at arm’s length. Those incoming letters interrupt her ordinary routine of private, defensive writing and force her to reckon with what she has avoided: she comes to see that the unsent letter she has been composing needs to be read, and that she cannot move forward until she finds it in her heart to offer forgiveness. In short, the returned correspondence turns private processing into a demand for confrontation, recognition, and resolution.