While it's true that we may not be able to observe the elemental building blocks of our own minds, metacognition is a real capability that people have and use.
To the contrary there is very good evidence that we pull arbitrary explanations whenever needed. See Gazzaniga's split brain experiments where people gave all sorts of reasons when asked why they did something and just could not know.
The fact that in extreme circumstances we pull explanations out of our asses (or even that we do so in non-extreme circumstances) doesn't mean we are incapable of genuine meta-cognition. No one is rational most of the times, but mathematicians can be rational for brief periods on limited subjects.
I think you're confusing cognitive processes with neurological processes. Of course we can reflect on actual cognitive processes such as forgetting or learning, and of course psychologists can gather evidence on these cognitive processes.
What we cannot do is make definitive claims about neurological processes and structures based on what we know about our cognitive processes.
"We can forget stuff" and "I believe I just forgot that" are very different things.
One is a general statement of fact, the other implies introspection of one's own individual cognitive processes. I have yet to see evidence that the latter is actually possible.
They are different things, but there is an entire scientific field that connects the two.
The phenomenon of people forgetting stuff is not subjective. It's not one person in isolation thinking they just forgot something. Forgetting is a phenomenon on which there is a lot of social feedback and repeatable experiments.
You seem to be denying the possibility of ever connecting science back to individual perception. Denying this makes any and all science completely meaningless though.
It's not limited to observations about ourselves. You could ask the question "but aren't you hallucinating?" about absolutely everything required to verify the outcome of a physics experiment for instance.
