The self appears to itself (and to other selves) as an absolute entity. Each of us believes that we are the 'same' person we were many years ago; we believe that the people we know are the 'same' people we've always known; we often believe that people of certain 'types' are always of that 'type'. It's unsettling to discover that someone we 'know' as such is different than we understood: a criminal, a pervert, a sinner; a kindness, a saint, an exemplar…
It's easy (philosophically) to observe that the 'self' constantly changes. None of us is the same person we were years ago, nor would we necessarily want to be. We grow and evolve. We are only the 'same' as a matter of historical trajectory or congruency: that which has 'this' name and 'this' job and 'this' spouse, without regard to actual behavior.
In the end (metaphysically), who we are is an irrelevancy that will be lost to time. How we are (for good or ill) — our relations to other; what we project forward through interaction — will persist. When I die, no one will remember my grandfather, but what he taught me will carry on with those I teach.
Descartes wanted to establish the unique existence of the self. Hume wanted to demonstrate that this 'self' was a complex and changeable entity, not a simple and solid one. Nagarjuna (in an entirely different philosophical vein) wanted to explain that this complex, changeable self was a pragmatic illusion that had use, but no ultimate meaning.
This is a deep dive. The question we all face is how much we can handle. Pick your level and take the rest on spec…