Need Help Understanding These Poems

"I Am Not I"
Juan Ramon Jimemez

I am not I.
I am this one
walking beside me whom I do not see,
whom at times I manage to visit,
and whom at other times I forget;
who remains calm and silent while I talk,
and forgives, gently, when I hate,
who walks where I am not,
who will remain standing when I die.

Who is this other?
Why is he only able to visit?