I don't think this is 100% true, but I think it's...oh, at least 20% true, perhaps much more. I think my mechanism for predicting the future impact of my present conscious thoughts is flawed (ie Stumbling on Happiness, or any consistent mis-prediction about self, like # of drinks consumed, junk food eaten, time it will take to complete a project, etc.) But I don't think it's pure rationalization, and the further an activity is from primal drives (sex, food) the more likely I am to successfully predict it, and so I think the more my conscious thought really matters.

One thing that really helps me have more belief in my conscious action is that rationalizations are not perfect - with time & practice, you can catch yourself in them. And they are not evenly distributed by action type. Sure I might have some hidden rationalizations (around death, my own abilities, other things that make me very uncomfortable), but there's just no way that all of the types of action I engage in have hidden rationalizations, such that my conscious model/predict/observe/revise process is flawed about everything.