The only way I can keep her off me is to hold her by the halter and keep my arm extended. The second I only hold the lead, she is right back into me trying to curve her head and neck in front of me.
You need to make it unpleasant(not painful or scary) to do the 'wrong' thing. Eg. I tend to take the tactic of randomly waving my elbow around while leading a pushy, crowding one. Not AT the horse, but if she gets in the way... Start out doing it consistanly, but switch to random once she gets the drift. They soon learn to stay out of your personal space. You can do this on or off lead. In the paddock you can act as if you're swatting flies - don't focus on her, but if *she* gets herself in the way & runs into the pressure, so be it. If arm length is not enough, use a stick or swing a rope or such. Be consistant at first & don't go after her with it, so she learns you're not out to get her, just that she needs to stay out of your personal bubble unless invited.
I have tried a tug and verbal "quit" with no improvement.
You aren't wanting her to come closer, so don' tug on her. Unless she's already learned what 'quit' means, you may as well say 'blah'. If you want her to learn verbal cues they need to be paired consistently with actual reinforcement or punishment - ie consequences, for her to learn the meaning. Eg. 'uh-uh!' to my horses mean they're going to cop some pressure if they keep doing what they're doing.
I have tried backing her up then as well (no luck).
Is it possible you've tried a number of things like that but given up too soon when she doesn't get it quickly, rather than being consistent & persisting? Is it possible your timing's out & you're not responding quickly enough? Backing the horse up can be a very effective method, but I'd be watching her, to do it as she *starts* to come too close, not waiting for her to crowd you. I'd also do it with energy & attitude, arms swinging(use as little pressure as possible, but as much as necessary to be effective) & while I wouldn't aim to hit her, if she doesn't get out of my way quick enough, she will get 'run into' by my arm, stick, rope, etc. I would do this each & every time she even thinks about coming in closer than arm's reach(or where ever you decide your 'personal bubble' reaches) without being expressly invited.
I would also be clearly rewarding 'good' behaviour, however fleeting, but make sure the 'bad' behaviour NEVER works for her.