Positive thinking isn’t always the panacea it’s cracked up to be. It all depends on context - when and why you use it. Sometimes it’s fine to fuse and really buy into ‘I am super-duper brilliant at parking’ or ‘I nailed that interview.’ But you can fuse to a point of toxicity - ‘I am such a great driver that I can drive when I am drunk,’ or ‘I am such a great surgeon I don’t need to follow the rules about not harvesting organs illegally.’ One Donald J Trump is a good example of fusing unhealthily with ‘positive’ thinking.
The question is this: does fusing with your thoughts help you live the kind of life you want and be the sort of person you want to be? So, if positive thinking works for you, go ahead but hold it lightly. More often than not, compulsive positive thinking is a way of masking darker, difficult feelings and emotions. And guess what? No good comes of that in the long run.