The 'I'-ness is what is most intimately you but also what is not you at all. It is as if the 'I' in the mind is borrowed, is on loan. What makes it seem like that? It peculiar absence, its being just exactly what you can't fix or grasp in any way. Something like Hadrian's anima vagula blandula - the weird recognition that the soul does not belong to you - but different as well because it is not separate. This is captured in the impossible idea that the is only one 'I' - not just for every human, but every sentient being. You can't make sense of this, you shouldn't even try to, but somehow you know it is true. Perhaps the best you can do is to realise that different logics belong to different levels of being - and then that there are no different levels of being, not even one.