Imposter syndrome is endemic in postgraduate degrees, especially doctoral. Many spend the first half wading in waters far too deep for them. Those who do not drop out manage to find the right model and the miracle is that they find their way to a well argued whole. Of course, they have not really reached any solid solution, but solved some minor problem based on previous work, but a good candidate will say something new. But to reach that rare inflection point a broad understanding, and a huge amount of creativity, and intuition is needed.
Can placebos last for significant stretches of time? I have taken antidepressants, two different kinds, over many years, and their effects are substantial over many years. If they didn't work I'd opt for other more toilsome methods of relief, and expensive ones like psychotherapy. But since they work and are relatively cheap it is a no brainer.