There's a fine line, and It's what makes diagnosing bipolar disorder very difficult for psychologists...because both of those are often in a positive, upbeat mood. What they have to distinguish, is if they display "risky" behaviors, like expensive shopping sprees (when they can't afford it), random business investments, gambling, prostitution, and taking drugs, for example. Because a person is who is in a positive, but also a healthy state of mind, will probably not do those things. Psychosis often occurs with full-blown mania, and because hospitalization is usually required, it's a lot easier to diagnose than hypomania, the lesser form of mania that many people have that goes unnoticed.
Positive thinking is having an attitude where you expect positive and good results. Visions of grandeur, or rather delusions of grandeur, at thinking more of yourself. You think you are more talented, likable, or powerful than you actually are.
Benedict Arnold 