"I" is my self-consciousness, the sum of my experiences, memories, thoughts, feelings, and perceptions, all my subjectivity and awareness.
My "I" is completely private, as is yours. I assume yours is like mine, but I have no way of knowing whether that is true.
My feeling about "I" is that it is the same "I" I have experienced throughout my life, and yet I recognize it is constantly changing in ways I understand and in ways I don’t.
Seeking to understand just what "I" is, is called (by philosophers and neuroscientists) the hard problem of consciousness. By comparison, other problems of brain and mind are easy.
The shortest period of time we can perceive is .01 second; I will call that the Present. Because we cannot distinguish these small blocks of time, we perceive time as continuous. "I" of necessity always lives in the Present; this is as true of dreams as of waking consciousness. During dreamless sleep, by definition, there is no consciousness, so "I" does not exist then. Of course, I am alive during dreamless sleep. So, strictly speaking, the presence of "I" is not necessary for me to be alive.
For a month during my hospitalization earlier this year, I experienced vivid subjective activity, while to the outside world I was mostly unresponsive. In some ways these experiences were similar to dreams, in other ways not, especially in the fact that, unlike dreams, I still remember these experiences in detail.
I very nearly died then, and this has left me thinking about death a lot. According to mortality tables, I can expect to live another 11 years. Then what will become of "I"?
I've often wondered how I will feel/react when the time comes near. Fear? Calm? As a general rule, I savor experiences. What about that one, as it approaches?
I'm thinking of "Me" as different from "I." "Me" is all of my qualities, characteristics, temporary or permanent, visible to others or not. What's "Me" are things that point outward from "I": personal traits, identifications, what I belong to, what I do or have done.
It is what I "am." For example, I might be tall, blonde, young, an introvert, female, a lawyer, Catholic, French, married, a mother.
You can see "Me," and for that matter "I" is usually focused more on "Me" than on "I" itself. "I" is preoccupied with "Me" from an internal point of view, as projecting "Me" out into the world, and as receiving feedback.
"Me" will be remembered and praised in my obituary and at my funeral, but "I" will not be mentioned, because "I" was never known to the world, and "I" will not be there at the funeral. Assuming "I" is not the same thing as my immortal soul, "I" will have disappeared and it will be as if "I" never existed.
But "I" did exist. And given that every moment is a Present moment, lived in those .01 second blocks, and that one moment is as good as another, those many moments "I" existed were as good as any other, and worth it, and sufficient.