Dimmesdale is a scared man because he is Hester's lover but does not want to admit it. He watches Hester suffer and face her sin while he hides his inside like a coward. However he suffers a great deal as well. .
"What can thy silence do for him, except it tempt him-yea, compel him, as it were- to open ignominy, that thereby thou mayest work out an open triumph over the evil within thee and the sorrow without".
We can see that he wants Hester to help him confess his sin, but she refuses so he will not get in trouble. He is unsure whether to confess or not because in his society, he is regarded as a person of high position and respect. If he were to confess that he committed adultery, everything he did would be gone. Still, if he doesn't confess, the guilt will destroy him from the inside.
Dimmesdale persuades the clergymen to allow Hester to keep Pearl on terms that Pearl is a torture for her to keep because Pearl is a child born from sin. She defends Hester and does all she can to help her. We also know that he has become weak and needs Chillingworth's help. These are some of the distinct clues that we see that Dimmesdale is Hester's lover.
"And I conceive, moreover, that the hearst holding such miserable secrets as you speak of will yield them up, at that lasty day, not with reluctrance, but with a joy unuttereable.".
Dimmesdale is torturing himself by whipping himself and starving himself. The guilt inside of him has already begun to kill him. His discussions with Chillingworth show that he believes everything will be fine as long as you repent in the end. It is ironic that Dimmesdale is friends with Chillingworth as they should be enemies.
When Chillingworth opens Dimmesdale's shirt, he is overjoyed. Obviously what he found should be the scarlet letter carved into his chest, it is the only thing that could bring so much joy to Chillingworth. This is disturbing as he has resorted to mutilation instead of confessing.