Note that the truth of this statement does not depend on any certainty about external reality, nor does it depend on certainty that what I perceive or remember is happening or actually happened.
It absolutely assumes a unitary conscious experience versus what increasingly seems to be the case, a bunch of narratives our brains thread into a cohesive story ex post facto.
Put another way, there very well may be hard limits to how much a human-like consciousness can understand itself.
That is exactly the reality I am asserting, whether or not they actually describe an "external" reality
And what I can be certain about is what my internal reality is.
And if you think I cannot be sure of that, I think I can be certain about I think my internal reality is.
And if you think I cannot be sure of even that, I think I can be certain about I think what I think my internal reality is.
It's perception all the way down, recursively. The reality is result of this taken to infinity.
There's abundant evidence to the contrary — at least as far as your recollection of what your consciousness previously experienced.
(IOW: Memory is extremely fallible.)
I said I can be sure of what I am recollecting, not whether the my recollection actually matches what happened.