I agree about 80%. The mind is a great tool and I believe most exercise is just about having the right mentality and you can achieve much more by just believing you can do something rather doubting yourself and then failing because of it.
I also believe the same about supplements though, and that most peoples "gains" come from them believing they should be able to lift more just because there on something like creatine or
beta alanine and not actually because the supplement is having a significant difference.