What happened to Dimmesdale in chapters 15-24?

In Chapters 15-24 of The Scarlet Letter, the Reverend Dimmesdale's secret sin begins to become such a burden on his soul that he suffers from the effects.
After having encountered Roger Chillingworth, who tells Hester that they are victims of fate and it is his role to be the fiend, Hester determines that she must talk with Arthur Dimmesdale. Knowing that he takes meditative walks in the forest, Hester hopes to encounter him, but she has no success until she learns that he will return from a trip one afternoon and she can cross his path in the woods. As she waits for the minister's approach, Hester tries to send Pearl off to play, but before she goes, Pearl asks why this minister holds his hand over his heart. Hester scolds the child. Then, Pearl decides that the minister places his hand this way for the same reason that her mother wears the scarlet letter. 
As the minister approaches, Hester calls to him. They both are awkward in this first encounter after seven years. Feeling rather awkward, they speak of insignificant things, but after a while, Dimmesdale asks Hester, "...hast thou found peace?" With a melancholy smile, Hester asks, "Hast thou?" "None! Nothing but despair!" he answers. He confides that he is miserable. Hester says, "The people reverence thee...and surely thou workest good among them! Doth this bring thee no comfort?" But the minister replies, "None!--nothing but despair! He tells Hester that whatever was good in him has become only spiritual torment now. "Hester, I am most miserable!" (Ch.16) Hester tries to encourage him by telling him the people have great respect for him, but Reverend Dimmesdale will not be appeased. He says he feels like a hypocrite when his "flock [are] hungry for the truth, " and they listen to his words as though "a tongue of Pentecost were speaking!" (Ch.17) Hester tells the minister his assessment of himself is wrong. "Is there no reality in the penitence thus sealed and witnessed by good works?" she asks him. (Ch.17).
Hester berates herself for having brought such woes upon the minister. It is very difficult for her to tell the Reverend Dimmesdale that Roger Chillingworth is her husband, but she feels that she must confess. This revelation causes the minister much distress to think that his sin has been exposed to the very man who would gloat over it. He tells Hester, "I cannot forgive thee!" But she insists that he forgive her, and he does, saying that Chillingworth's sin is blacker than theirs. Then, the minister worries that the old man will reveal their secret, but Hester contradicts him, stressing that Chillingworth has a secretive nature, and will seek other ways to avenge himself. Hester suggests that the minister go into the wilderness or leave the country. Dimmesdale protests that he cannot leave by himself; so, Hester promises to accompany him across the ocean. They both feel hope resurging in their hearts. In fact, Hester removes the letter from her bosom, and she tosses it. But Pearl refuses to come to her until her mother replaces this letter. Unfortunately, Dimmesdale does not relate well to the child who washes off his kiss in the brook.
After Dimmesdale agrees to leave the country, he feels as though he has been freed from his hypocrisy. Further, he has the strange urge to speak something "strange, wild, or wicked" to the people that he passes on his return home. When he refuses his medicine from the physician, Chillingworth becomes suspicious. But, nothing is said as the Reverend Dimmesdale retires to prepare his Election Day sermon. With the arrival of the New England holiday, the minister has a newfound energy, and Hester observes a different man from the one who talked with her in the woods. When he passes, the minister does not recognize her and seems beyond her reach. "Mother, was that the same minister that kissed me by the brook?" Pearl asks her mother. 
Whereas Dimmesdale has felt himself a pawn of fate, after talking with Hester, the minister now feels that he is not powerless to control his life. It is a different man who passes Hester and Pearl, but doom awaits him, Hester realizes because the captain of the ship sends word to her that Chillingworth will escort the minister onto the ship. In the meantime, the minister delivers his sermon. Weakened from the exertion of his impassioned speech, the Reverend Dimmesdale falls forward. He calls to Hester and Pearl, who stand by the scaffold. As if impelled by fate, Hester and Pearl come forward. Chillingworth catches hold of the minister, telling Dimmesdale that it is only by being on the scaffold that the minister can escape him. Then, the Dimmesdale tears open his robe, revealing "something" on his chest. He falls and is supported by Hester as he asks Pearl to kiss him. When Pearl does this, "a spell is broken." Dimmesdale dies after thanking God for his mercy in allowing him to confess and save his soul.