My intro to psychology classes was one of the most valuable classes I ever took, just with the way it systematically shattered my own notion of how much I could trust my own notions of perception and thought to be a rational and accurate reflection of reality. I definitely had a notion of how irrational “people” could be before that, but of course I somehow thought I was above all that.
Of course, we also must be careful here because you're using your own faculties to judge the content of the psychology class (as were the psychologists who produces the content you were learning). Skepticism falls into special pleading, because in order to take a skeptical stance toward the human intellect as such, one must somehow transcend the human intellect to be able to make those sorts of judgements [0].
I would also not say that we are inherently and constitutionally irrational. I would say rather - to use the old cliche - that the intellect's facility is like a muscle that needs to develop, to grow, and to be conditioned to become strong. I would also say that some have greater capacity and potential on constitutional grounds.
Furthermore, the cultivation of virtue is essential, as errors of reasoning are shaped by our vices and not just cognitive limitations or whatever. Indeed, the more intellectual power someone has, the more essential virtue becomes, lest the intellect destroy itself with rationalizations and abuse [1].
[0] Of course, what constitutes correct reasoning is a teleological matter. Otherwise, there is no reason to favor one conclusion over another or any conclusion at all.
[1] A coward of high intelligence will the rationalize powerful. In Chomsky's view, for instance, the overwhelming majority of intellectuals have historically acted as servants of power, rationalizing the status quo, and manufacturing consent. Or as Adam Zamoyski said pithily when discussion Napoleon's relationship with French intelligentsia, intellectuals are mostly tarts for power.