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

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.