The Bible doesn't teach "positive thinking". The Bible doesn't teach "negative thinking". The Bible teaches TRUTHFUL thinking.
"Finally, brethren, whatever things are true, whatever things are noble, whatever things are just, whatever things are pure, whatever things are lovely, whatever things are of good report, if there is any virtue and if there is anything praiseworthy-- meditate on these things."
At best, "positive thinking" is a fleshly attempt to will your way to success, like the self-image psychology technique of "reprogramming the subconscious".
At worst, "positive thinking" is the equivalent of occult visualization.
The most famous advocate of "The Power of Positive Thinking" was the 33rd degree Freemason, Norman Vincent Peale, the founder of Guideposts magazine. His protégé, Robert Schuller, carries on the legacy of this false teaching with his "Possibility Thinking".
Christians should advocate trusting in the Lord Jesus Christ and not in SELF.