That 'narrow slice of reality' model only works if you first assume there is an objectively existing 'truth' out there apart from experience. You are then like a philosopher in Plato's cave who finds a way free and goes towards the light, exclaiming, 'everyone, what you are seeing is an illusion!'

But what if the light is itself another version of the shadowy cave interior? If you assume an ultimate 'truth' then there is no way of knowing if you have arrived at it. You must always question whether this 'truth' you have found is the ultimate truth.

Solution: assume instead that all experience is valid. And that if you find that the experience you had then was, as you see it now, an illusion, that does not invalidate your 'old' experience. It simply makes your perspective now different from your perspective then.

Take care!