There are many ways to diagnose and treat our psychological problems.
For John's problem a neurobiologist would pinpoint his problem to a biological source. For instance he could have a chemical imbalance, which would be causing the depression.
Many of John's forgetfulness problems could be rooted to his subconscious thinking. As a Psychoanalyst you would look at his childhood and his current life and really focus on his close relationship with his mother. John could subconsciously be trying to forget things, like his date, so that he can stay at home with his mother.
As a Behaviorist you would assume that John has had negative associations with authority figures (his boss/mother). John possibly could associate his boss yelling at him as a personal insult rather than constructive criticism. When John was yelled at as a child it could have been primarily to insult him rather than to motivate him. John may have also never been disciplined as a child when he forgets things; so now, when he forgets things, he knows that there aren't any harsh consequences (like being kicked out of the house).
A Sociocultural Psychologist would look for things about John that would isolate him from culture he lives in. The Psychologist would look closely into the fact that he still lives with his mother and that he has a serious problem with forgetfulness.
The first thing a Neurobiologist would do is find out precisely what kind of problem he has; whether it be with the nervous system or a chemical imbalance. The main way that patients are helped is with prescription medication. This medication would cure the physical problems that John is having.
A Behaviorist would work a lot on teaching John new associations. For every event that John forgets he should be punished, but...