How does Dimmesdale feel as the much respected minister in the community and why doesn't he thrive among the people who so admire him?
Unlike Hester, who has violated a law of society but not a personal moral code (she is the one who claims "what we did had a consecration of its own"), Dimmesdale has violated the core of his being. Hawthorne's description is telling: "In no state of society would he [Dimmesdale] 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."
When Dimmesdale violated an central component of his faith, he lost the structure that had been supporting him. Much like Hooper in "The Minister's Black Veil," his acquaintance with his fallen humanity makes him more understanding of other's failures, and better able to minister to them. But none of this external approval could change that fact that he no longer approved of himself; without that approval, there is no thriving.
Dimmesdale's guilt has overcome him. He continues to preach and counsel his congregation, but in his sermons and actions he degrades himself and talks of his guilt in various ways. The problem is that the people of his community see his degregation as being self-imposing of the sins of the people of the community as his own. The people do not understand that he is confessing his real sins and preaching to try and absolve himself. He cannot, with a clear conscience, be true in his preaching when he has not confessed his sin with Hester.
In addition, Dimmesdale's sin cannot be forgotten. Every day, he must suffer by witnessing the the burden under which Hester and Pearl struggle. Because of his awareness of their difficulties, feelings of guilt, and recognition of himself as a coward/weakling/hypocrite, he is never allowed a moment's peace. Dimmesdale's physical deterioration is due to his emotional deterioration.