I'm not sure that Dimmesdale does not remain true to himself ... it's just that it was very destructive. This somewhat lengthy quote highlights that point: " Mr. Dimmesdale was a true priest, a true religionist, with the reverential sentiment largely developed, and an order of mind that impelled itself powerfully along the track of a creed, and wore its passage continually deeper with the lapse of time. In no state of society would he have been what is called a man of liberal views; it would always be essential to his peace to feel the pressure of a faith about him, supporting, while it confined him within its iron framework."
Dimmesdale's self was intimately tied up with his sense of religion, supporting him in its unbinding "iron framework." Just as Hester remained true to her freedom from the censure of the Puritan community, so Dimmesdale remained true to the framework that supported him.
Many people find Hester a much more sympathetic, true-to-hserself character; I find Dimmesdale to be as true to his values as Hester, although the consequences of this loyalty are not at all admirable in many people's eyes.
Pearl is Hester's torture because she is a constant reminder of the sin she committed with Dimmesdale and she is also an unusual "elf-child". Pearl often reacts in strange ways to others and to her mother. For instance, when trying to determine custody of Pearl, Governor Bellingham asks Pearl where she came from. He obviously expects her to answer that she came from God. Instead, she answers that she was plucked from the rosebush outside the prison door. This shocking answer almost makes Bellingham take Pearl from Hester. It is onlythrough the pleading of Dimmesdale that she is allowed to keep Pearl. Pearl also has a fit when Hester tries to remove the Scarlet Letter. However, as any mother knows, Hester loves Pearl. She learns from Pearl about the consequences of her actions and she remains strong for her daughter. during the course of the novel, only Hester and Pearl really stay true to themselves. Hester never tries to hide her sin, and Pearl really has nothing to hide. Both Dimmesdale and Chillingworth have hidden sins that eventually destroy them.