I can be absolutely certain of my perception and recollection of what my consciousness is experiencing and has experienced.
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.
> nor does it depend certainty that what I perceive or remember is what is
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.