Just an attempt; may not match each statement of your puzzle.
The Pluto Planet?
A mysterious thing I am
We know the planet and few facts about it. But, there are many mysteries about it yet to be reveled.
So you say now I am not what you used to say I was? You've kicked me out of the group you used to say I was a member of?
It was used to be called "Planet". Now, it is shifted to "Dwarf Planet" category.
Well, without me the other members of the group are eaten, ungrammatically.
While formation of solar system, many other small objects were merged in bigger objects. This one survived. That said, other small objects were eaten up by large objects.
But say what you wish; it makes no difference to me.
It is material, non-living thing. It makes no difference to it what we call it or in what category we put it.
I was here long before you ever existed, and I'll probably be here long after you're gone.
Of-course; isn't it? Considering "you" as I individual or a human, the statement is true.
To date I have been mostly unreachable for you,
"mostly unreachable", yes. We have not yet landed on it.
though lately you have been doing some probing into my business.
Human have sent many un-manned space ships to investigate solar system and other parts of universe. I guess "probing" is pointing to it.
Really, it's been less than half a year that you've even known of my existence, though you would say it's been 90.
Don't know. Not sure if this is about "year of Pluto". I will investigate further.
I don't mean to sound bitter about any of this; I'm really very ambivalent.
Humans have contradictory ideas about it; some say Planet and some say Dwarf Planet.
If I sound cold, well, that's because I am. Very.
It is cold, very cold planet (or Dwarf Planet).
We are distant and different, you know.
Not sure but if we compare Pluto with Earth, yes; they are distant and different.
What is bright and blinding for you is only a glimmer for me.
The Sun that looks very bright and is blinding from Earth is glimmer on Pluto.
And yet in a way, I am in your element.
Don't know. May be chemical elements forming Pluto and Earth are same?
You named me after one thing and then named another thing after me; the second thing being the first in reverse.
Partial answer. Planet was named Pluto after "Roman god of the underworld". Don't know what other thing that was named after Pluto.
Knowing me well, as I have said, would be difficult for you.
Not sure but to know about this planet more, human need to spend too much money. Distance is too long; it is hard to reach and come back from there; we do not have that technology yet. That is why, it is difficult.
And maybe you don't want to dive deep into learning about me right now.
Not sure but "dive deep" into the Pluto will kill (obviously) the human.