We had similar problems with our young cob, he'd been treated very roughly in the past and had issues because of it.
Have you tried smearing molasses or similar on the bit? Might help her to accept it..... We also kept the head collar round his neck but didn't tie the rope to anything as he used to back up, panic and rear. Just had someone walking backwards with him. Also a very sticky, minty molasses lick did wonders

We had problems with getting a bridle on, picking up feet, putting on rugs and standing to be mounted. It just took time, patience and Positive but not rough handling. 18 months on and he is an absolute star, he has the odd moment when in a new situation but my daughter can now sit on a milk crate by his back legs while brushing his feathers. You can put his rugs on and off in the field and almost always stands to be mounted.
Hopefully with time and regular handling she'll come good, we had some moments with our lad when I thought he'd never improve but the change in him is truly amazing and we all love him dearly ( even my anti horse husband)
Just a thought....have you had her teeth checked? Might account for the biting problem