Your argument is reminiscent of Wittgenstein's discourse on private language:
The idea of a private language was made famous in philosophy by Ludwig Wittgenstein, who in §243 of his book Philosophical Investigations explained it thus: “The words of this language are to refer to what only the speaker can know — to his immediate private sensations. So another person cannot understand the language.” This is not intended to cover (easily imaginable) cases of recording one’s experiences in a personal code, for such a code, however obscure in fact, could in principle be deciphered. What Wittgenstein had in mind is a language conceived as necessarily comprehensible only to its single originator because the things which define its vocabulary are necessarily inaccessible to others [emphasis added].
Immediately after introducing the idea, Wittgenstein goes on to argue that there cannot be such a language. The importance of drawing philosophers’ attention to a largely unheard-of notion and then arguing that it is unrealizable lies in the fact that an unformulated reliance on the possibility of a private language is arguably essential to mainstream epistemology, philosophy of mind and metaphysics from Descartes to versions of the representational theory of mind which became prominent in late twentieth century cognitive science.
This connects up with the conceptual problem of other minds:
Thomas Nagel once wrote:
The interesting problem of other minds is not the epistemological problem… It is the conceptual problem, how I can understand the attribution of mental states to others. (1986: 19-20) [emphasis added]
Bilgrami agrees (1992). Some philosophers go further than Nagel and insist that the conceptual is the fundamental problem; others see little in it (Hyslop 1995). How one understands this problem is a matter of some contention (Gomes 2011). What all agree is that the problem is associated with the work of Wittgenstein, and in particular section 302 from the Philosophical Investigations:
If one has to imagine someone else’s pain on the model of one’s own, this is none too easy a thing to do: for I have to imagine pain which I do not feel on the model of pain which I do feel. That is, what I have to do is not simply to make a transition in imagination from one place of pain to another. As, from the pain in the hand to pain in the arm. For I am not to imagine that I feel pain in some region of his body….
The gist of it is that we do not learn many, or even most, words purely "on our own" but in response to our social environment. We have direct, internal knowledge of this fact (see Kant's "refutation of idealism" in the first Critique for a broader example of the deeper principle). So even if we suppose that our consciousness were the only consciousness to exist, we will still end up having to assert the existence of something that is not our consciousness, and which hence is not us (on the solipsistic model), although this one other being might be thought to exist just so long as we exist anyway.
Pragmatically, let us suppose we find ourselves believing in other minds and attributing them to the entities we observe speaking and writing around us and to us. Imagine that our first exposure to the other-minds problem is not occasioned by our own practices of doubt, but due to questions coming from these outside quarters. So we are imagining that a possible philosophical zombie is the one teaching us about the possibility of philosophical zombies. If we have no reason to believe that these beings are not zombies, we have no reason to believe them when they "say" that such zombies are possible. So what basis do we have for believing in such a possibility? If I tell the hypothetical zombie, "I directly know, beyond doubt, that you are not a zombie, although it is logically possible that you could have been, so to say," they have nothing to gainsay us with. The maybe-zombie might reply, "You don't know that," but again, they are not in a position to defend this claim pragmatically.
Now, are we being honest with ourselves if we say that we know that we are not the only minds in the world, that there is an external world (whatever that means), etc.? It might be that the concepts of possibility, uncertainty, knowledge, etc. are all amorphous enough that at the end of the day, the issue of our self-honesty, here, goes deeper than that. "I know what knowledge is," might be rephrased as, "I am directly aware of what I use the word knowledge to refer to in general, maybe not before and always, but at least at this moment: I stipulate that..." The skeptic who always asks, "How do you know?" tries to avoid playing an assertion in this language-game, but if you rejoin them with, "What do you mean by, 'How do you know'?" then the game is up. Not that you win and they lose, or even quite that the game goes to default, but more as if the game itself is seen to have been an illusion from the start. Conifold claims in a comment that:
If it is surprising [that our minds might contain ideas that they cannot understand] it is because of this historically inculcated rationalist idea that before mind does something it conceives it, and then wills it to persist, while being self-aware the whole time. And that is not how minds we know function at all. They mostly have no clue what is really going on in them even when they are aware of it.
This whole line of reasoning presupposes that we have real evidence about history, the inculcation of ideas, "how minds we know function at all" (knowledge based on what, exactly?), and a cynical overgeneralization about the scores of billions of human beings who have ever lived. We can grant that the concept of clarity is not itself clear to everyone, all the time, except that we can also grant that there are "possibly" no such things as concepts (as such) or even beliefs, for that matter. If no one believes anything, then no one has any false beliefs, and no one ever misunderstands what they believe or how they believe it. "Am I always dreaming?" ends up interchangeable with, "Am I always awake?"
Addendum. Still, for all that, I would also like to mention the idea that we might have all possible concepts preloaded into our minds from the get-go. I don't recall if it was John Searle or Jerry Fodor who suggested/argued for this claim, but it's out there. In that event, there might be concepts that we don't consciously understand as yet, but which we might come to understand some day. You say in the OP that you think some notions will remain mysterious for you perpetually, but how certain are you of that? If you are immortal, can you say with so much confidence that after a trillion years of reflection, you wouldn't grasp physics even beyond the kind we are working with in this era?
