The article, mental illness, by Mayo Clinic suggests that mental health problem refers to a wide range of mental health conditions — disorders that affect your mood, thinking and behavior. Examples of mental illness include depression, anxiety disorders, schizophrenia, eating disorders and addictive behaviors.

Many people have mental health concerns from time to time. But a mental health concern becomes a mental illness when ongoing signs and symptoms cause frequent stress and affect people’s ability to function.

Mental illnesses, in general, are thought to be caused by a variety of genetic and environmental factors:

Inherited traits. Mental illness is more common in people whose blood relatives also have a mental illness. Certain genes may increase your risk of developing a mental illness, and your life situation may trigger it.
Environmental exposures before birth. Exposure to environmental stressors, inflammatory conditions, toxins, alcohol or drugs while in the womb can sometimes be linked to mental illness.
Brain chemistry. Neurotransmitters are naturally occurring brain chemicals that carry signals to other parts of your brain and body. When the neural networks involving these chemicals are impaired, the function of nerve receptors and nerve systems change, leading to depression and other emotional disorders.

In the novel the scarlet letter, Dimmesdale has a mental problem. Dimmesdale's condition is psychosomatic. This means that his physical ailments stem from a very disturbed state of mind, and that the latter manifests in physical symptoms that connect to the situation that he is going through. In his case, guilt has made him weak to the point of consistent depression.

Dimmesdale's sin has caught up with him and it is affecting his present along with his future; his secret sin is eating him up. He is beating himself up because he has kept it locked inside of him when he should have openness about his sin.