Alright alright I got this one! The reference to Constance Contraire drew me in - haven't thought of that series in a while - but the hint really gave it away for me. (Spoilers for the series ahead.)
Who is the narrator?
Cassandra, aka Evanlyn, from John Flanagan's Ranger's Apprentice series.
Am I stubborn? Just as much as Constance Contraire.
Evanlyn is stubborn - it's one of her best traits, as well as one that gets her into trouble.
Steady fingers on the bridge,
Referencing book 2, The Burning Bridge, and her role in it.
Just as steady as my mind when it's made up.
Again, a reference to how determined she is.
I love one whose weapon is that of the just.
She is in love with Horace, who is a knight and fights on the side of justice (with a sword, commonly regarded as a weapon of justice).
They may have taken their vows against me, but I'll save him 'cause he's to me like glittering garland is to silver-fist
Not sure about this one. Possibly referring to the fact that Will chose not to enlist in Battleschool, instead staying with (making his vows to) Halt and the Rangers? Unsure who silver-fist is.
We are divided, awkward and shy as he may have been so long ago.
Will was quite awkward and shy as a kid, and Evanlyn and Will are now separated by location as well as somewhat by circumstance.
But my love, he is just as outgoing as always.
Not sure if this is referring to Horace (romantic love) or Will (platonic love)? Both are quite outgoing by the second half of the series.
I used the same weapon of my daughter,
Both Evanlyn and her daughter Madelyn favor a leather slingshot as their primary weapon.
Whose main character trait was inherited from me.
Madelyn inherited her mother's stubbornness, which causes trouble for her as well as for her parents in book 12 (The Royal Ranger).
Title - "which one is my real identity?"
I assume this is referring to the fact that the narrator has two identities - Evanlyn and Cassandra.
And the hint:
We are 12, the first a haunted ground/ The second is like the second line/ The third is entombed in ice/ The fourth is those who own a special pendant/ The fifth is a misunderstood healer/ The sixth is against all odds/ The seventh is a bargain for a big man's head/ The eighth is about mentor's home/ The ninth is the mentor's danger/ The tenth is a noble noble-man/ The eleventh was lost long ago/ The twelfth is the subject's obdurate child.
The (12) books in the series, referring to either the title itself or the plot of that particular book: The Ruins of Gorlan, The Burning Bridge, The Icebound Land, The Oakleaf Bearers (though I, and other US readers, know it as The Battle for Skandia), The Sorcerer in the North, The Siege of Macindaw, Erak's Ransom, The Kings of Clonmel, Halt's Peril, The Emperor of Nihon-Ja, The Lost Stories, and The Royal Ranger