I also agree you are mismatched.
When you slap her on the neck it means absolutely nothing to her. When she turns to bite you whilst leading you need to be quick with your hand and meet the side of her muzzle with a good open handed slap or carry something sharp like a large nail and ***** her with that when she tries it on.
Much the same when she tries to bite your leg, do not move your leg away but use your toe to give her a good kick on the muzzle.
You really need a trainer.
Carrying a horseshoe nail with just tip showing between the fingers of a clenched fist that is holding the halter rope is how I do it, never a large one. And in this case, I doubt that the OP is going to be able to do this to where horse is corrected. Rather it will result in a wound.
The whole purpose of the hidden horseshoe nail is to let horse think it has corrected itself, and even then, if use a large nail, and this is happening while leading means more confusion in hand placement for OP, just don't think it will go well.
Also having trouble visualizing exactly where slap is coming from, OP should be on near side leading off of her right side, so horse bites at her, is she stopping and then facing horse to slap with right hand, reaching across with left hand to slap or???
Either way, waste of time.
For this, and mind you I rarely have had to deal with this, was taught better from the beginning, but you get horses that have been spoiled or ruined due to mishandling, my method of leading one does away with a lot of problems.
I lead with my shoulder at throatlatch, left hand holding coils of lead rope, right hand about a foot from underside ring on halter, where lead rope is snapped on, I either have my forefinger resting along the rope with nail toward horses muzzle, or will turn my hand so thumbnail is, and if horse comes in, I shift my hand to let it bump into my fingernail.
I never acknowledge that has happened, no talking, nor even looking at horse, it has corrected itself and thinks it had better not do that again.
If the bump into fingernail doesn't get message across, I will turn hand and give a quick jab with thumbnail into soft side of muzzle just above lips, again no talking to or looking at. Correction is made and understood. And problem is stopped.
As for the turning to bite while up? Why on earth is your rein loose enough horse can get head that far around to begin with?
Get a trainer OP.