You need to be more firm with her. If a quick yank doesn't remind her who's boss, get a lead rope with a chain on the end. Thread it through her halter (have someone show you how) and use that to lead her. A lot of horses won't try anything after that because they know it's there--you never have to use it. Don't pull on it or keep it tight; use it just like a normal lead rope. When she goes to pull you, give it a yank just like you've been doing. The chain will give you extra force that you didn't have before. After she pulls and you stop her, make her stand there until you decide to start again.
For all those who give thumbs down at the mention of a chain, tough. People must understand the difference between insisting upon respect (in which you use a chain only as intended in a situation when a regular lead rope didn't work and the situation could be dangerous) and abuse (such as if you were to beat her with the chain for no reason).
My sister's 5 y/o WP mare can be a little difficult, but if we use a lead rope with a chain, she's perfectly behaved (and we don't have to use the chain--she knows it's there). You'll rarely see a halter or showmanship horse at a big show showing without it. It's only "mean" if used incorrectly. If used correctly, it allows for more precision and control than a regular lead rope.
If the mare is pushy in other ways (like rubbing on you, crowding you, etc.), smack her when she invades your space. She must learn that she is the horse and therefore must listen to you. You're in charge. If you have to, carry a whip. I do not abuse my horses, but if they go to dancing on my toes or knocking me over, you can be they'll get smacked. As a result, they've got excellent manners and need very few reminders.