It would be easier if we either remembered things or just simply forgot them. But sometimes things just do not enter our minds.
If memory is one and forgetting minus one, then there is a zero. A nothingness of things that neither are nor are not.
Sometimes a thought trips over a nothingness and we feel a moment of unease, like but not like the sensation we have when we think we have forgotten something.
For that sensation still has an echo of the object, whilst this is wholly of the void. No matter how much we strain we can never find the lost memory, it never existed in the first place.
What is is but an infinitesimal in what is not.
And that is why your birthday card is still sitting on my mantelpiece.