Every time she goes to nudge you, give her a quick sharp elbow in the muzzle. I don't like to slap as some horses become shy from only one. And then you have yourself another problem.
Work her in the halter, teach her to keep out of your space. Backing up, turning on the haunches away from you(Never towards you!). Turn on the fore. Lots of walking and halting, then trotting and halting. The most important thing here to to be patent and relaxed. It may take one session it may take 10. But a horse is never too old to learn.