I think she did not realize that, when she wrote the books, how much Mr. Banks was her father. When they got to the movie, decades later, she did not want to have her father portrayed as the mean guy he was in the books and, from all accounts, in real life.
Oddly, after her father died, she did live a charmed life. Her mother's family was well off and well connected. She went to a boarding school in England and then lived a very nice life for many years to come.
I thought it was twisted how she changed her name from her birth name (Helen) to Pamela (I have not seen any reason why she took that name) Lyndon (her real middle name) Travers (her father's first name). She was never married, yet she wanted to be called Mrs. Travers. She wanted to be married to her father?

It was like she wanted to hide from her true identity (Goff).
By all accounts, she was a horrible mother to the boy (some say she adopted both of the twin boys but only kept one - records are unclear) she adopted.