Although Positive Thinking has certain merits, it has become evangelized by New Ageism, at the expense of understanding how Positive Thinking fits into a larger picture, for better and worse.
Yin & Yang
Positive Thinking's flaws are found in its duality — which is a false dichotomy.
Positive Thinking is the technique of biasing one's thoughts into a positive nature instead of a negative nature.
What Positive Thinking fails to grasp is that these inner polarities are in themselves defunct at a spiritual level. Positive mental charges are just as ineffective as negative mental charges. If one is not aware of this, they risk converting negative charges to positive charges.
But the true issue remains: the charge.
Positive Way of Being
More important than Positive Thinking is possessing a Positive Way of Being.
The Heart is the only part of ourselves capable of neutralizing charges.
A positive way of being is one in which the Heart is open and capable of processing all polarity the person comes across, both positive and negative.
For this reason I like to say that I don't want to "feel good," I want to Feel True. Feeling truly grants us the ability to feel, integrate, and neutralize the negatives. If we focus rigidly on "feeling good," we risk avoiding the negatively-charged shadow parts of ourselves that are trauma-charged precisely because they did not receive Heart-attention in the first place.
Negative Way of Being
A negative way of being is one in which the Heart is closed in part or in totality.
At a minor degree of closure, this leads to the toxic New Age trend of focusing only on positive thoughts while disavowing/avoiding/rejecting negative thoughts. Although not obvious on the surface, the reason this is an issue is because negative thoughts typically arise from unresolved wounds and trauma. To reject them and bulldoze over them with "Positive Thinking" is to re-traumatize oneself and perpetuate one's inner suffering. These lost parts of ourselves are known as our shadow.
At the extreme, heart-closure leads to narcissism. In this scenario, all thinking is self-serving and does not accommodate others.
New Age Narcissism is uniquely bothered by shadows and charges and will claim to integrate the shadow, while in actuality paving over shadow elements with a sense of spiritual superiority derived from Luciferian False Light. This contradiction stems from the fact that non-Christic Masculinity/Mind cannot perceive the full extent of the human-spiritual shadow. An awakened but un-unionized mind does not understand that negative emotions and unresolved wounds are not the only shadow aspects of this realm, but that there are also larger, utter, anti-life inversions and tactics being employed which are also shadow elements.