Has she not had any good experiences with her mouth? A lot of people are against feeding horses treats, but if you give her a treat from your hand, she will see not all experiences messing with her mouth are bad or scary, lol.
But you said she was a yearling too, so she may just need to get used to it. And maybe even bond with you more? An old saying I heard, was the horse trusts you when you can touch between their eyes,( as they can't see you hand), without their head moving, if they stay relaxed.
When I first got Bo, he was horrible about tossing his head up for anything! He was a two yr old. I worked John lyons in the round pen, to build trust, work on lowering his head at the poll (he did fine with a halter) but not with the bridle. I realized he just wasn't used to the movements of my hands around his face like that, which trust plays a major part. So I started with just my hand, moving it slowly, then as fast as I would with the bridle, then I just practiced taking the headstall on and off, then finally accepting the bit. (not moving to the other step until he was relaxed and fine with the previous. Lol and it definitely didn't happen over night)
But Bo is used to me handling his mouth. He knows it could be wormer, or a bit, or just my hand, or treats. We have a better bond now, and he won't even raise his head when I put the bit in! Before, my trainer had to tie his head down, but now, I don't have too. I hope this helps!
Lettin' the cat outta the bag is a whole lot easier than puttin' it back. -- Unkown