[Galadriel55]

The riddle also reminds me of Feanor.
Twilight - Miriel was born in ME when there were only stars
Her path did end by western shores - she died in Aman, ie in the West
After glory came the loss - Feanor brought the glory of Valinor to an end, and loss could refer to either the Trees, the Noldor going away, the dtolen Silmarili, or something else that I can't think of at the moment.
Close to the King was she of late - Miriel was Finwe's wife
I have been thrown into the war - Feanor started the war
in blood I've seen the Dark One's fall - I don't know how that works
when Men of Light have earned his hate - I don't know how this fits either, since Men didn't exist yet

I've been a friend to all the guards
of light - of peace - well, he was friendly to the Valar once, and he stored light in the Silmarili that are very hard to break - he put a "guarding wall" around the light
till I betray'd
them in the vicious nightly raid - he attacked the Teleri
when fear was piercing all their hearts - when Morgoth destroyed the Trees and killed Finwe
They came and went - I have remained - the Silarili still exist, although they are separated.