Although the most common answer to the psychological effect that Parkinson’s disease has on individuals will be that of patients, there is also the psychological effect upon those who are related by blood to those patients as well as the psychological effects upon caregivers.
Then there is the effect of fear, ignorance, cruelty, sympathy from the general public.

Depression in PD the most obvious effect. As depression is also a symptom, an early symptom of the disease and a parallel effect of having the disease, it is probably the most common psychological symptom. It is treated differently.
http://parkinsonsfocustoday.blogspot.com/2009/03/questions-about-parkinsons-disease-part_13.html

SAD is another psychological type symptom. Don’t forget that many PD patients do not or cannot get around as easily and do not venture out into the sunlight and because of the disease may be vitamin D deficient anyway.
http://parkinsonsfocustoday.blogspot.com/2009/03/parkinsons-depression-and-seasonal.html

Fear is another symptom: fear of people knowing, being ashamed of the effects of the disease, the fear of people’s reactions to the disease, the fear of not having the support of family and friends sometimes because the patient doesn’t have a support system in place or because they really are alone. The fear of losing a job because of loss of skills or because of stereotyping. The fear of dying. Fear of falling and being injured. The fear of losing one’s driver’s license. Plenty of reason for fear.

Anger: anger at having the disease. Anger that there is no cure. Anger as a side effect of some meds…sometimes taken out on loved ones. Anger at the loss of strength and mobility.

Frustration: see above and add frustration at not being able to perform simple tasks such as buttoning a shirt, putting on socks, not getting enough sleep. Frustration that your physicial sees you as a disease and not a person with a set of symptoms which may vary considerably from the last PD patient the same doctor saw the day before.

Sleep disorders: are sometime psychological although there are often other reasons in PD

One problem with depression in PD is that it is somewhat contagious to family and caregivers. This is why the depression counselling often involves trying to restore self worth.

With muscles don’t work as well, the face may not express emotion and the speaking voice may become a monotone – this is a mistaken sign of depression – rather than a sign of not enough facial and voice/breathing exercises. Smiling is a great facial muscle exercise and serious yawning is just one of several throat relaxing exercises.

Caregivers have a confusing role. Often the cargiver is a spouse and there is a certain loss to deal with. "In sickness or in health" didn’t conjure up this vision when the vows were taken. That’s the great thing about couples who survive long enough, there is an underlying love and companionship which develops to carry both parties through this journey. But there is a loss and that is depressing…make no mistake.

Family is sad in many cases because they don’t know what to do. They also don’t realize why that person might have changed. They feel overwhelmed by the information and why the fact that there is no cure.

The public response to a disease where someone does not behave or look "normal" is fear, aversion, excessive-compensatory display of sympathy, bad taste … you know the litany. Sometimes it brings out the best in people – sometimes the worst. Employers fire people long before they cannot do their jobs or do a job for the company because they are afraid or because PD is in limbo for federal protection.

The best way to deal with the public if you are a patient is humor. We all need to laugh.

brain gets slow, and memory goes rubbish as well

Use your imagination. That should already be a source of information. Imagine that you cannot hold anything in your hands and that you cannot move around without shaking and a risk to fall over you own feet. How would that psychologically affect you?
I’m not being cynical here. I think it is an exercise to do before diving into more profound and specialized information .

Severe depression is very common– in fact, it’s nearly universal in Parkinson’s patients.
Imagine deteriorating a little at a time. You get so you can’t button your shirt. Then comes the tremor. Along comes the danger of falling. After a while you can’t dress yourself, bathe yourself, toilet yourself, or speak loudly enough to be heard. Pretty soon, you’re on a pureed diet and need to have thickened liquids to avoid aspirating stuff into your lungs and getting pneumonia. You may end up on a feeding tube. You are guaranteed to end up in a wheelchair, unable to manage to control your saliva. You feel as though you are a burden to your family. You begin to lose your friends.
Not fun.