Hmn, I'm going to take a stab at this. Galadriel? I'm not convinced I'm right, but I might as well try.
For so long time I'm here - in Beleriand/Middle-Earth
though not from start of times. - of course not.
In greatest treachery
I at first took my part. - Following the Noldor out of the Undying Lands
I saw how Darkness comes, - Morgoth in the 1st age
I saw how it departs. - Morgoth defeated
I saw how it returns, - Sauron returned
I saw the death of stars, - The Silmarils lost, and/or Gil-galad killed
I saw corrupted hearts. - Isildur/Saruman/Smeagol corrupted by the Ring.
Once They will come to me - the fellowship
to undo treachery - when she refused the Ring, the ban was lifted
at that day, it all ends - the diminishing of the elves in ME?
I'll die by my own hands. - passing into the West, at last