She probably charges because she knows now that she can move you and intimidate you. Lead horses move lesser herd mates by applying pressure. The lesser herd mate moves out of the way.
Funny enough, I pass a small "zoo" on my way to town. These people collect different color horses (all their horses are a different, odd color). They have a cow, a llama, two minature horses, a mini donk and a miniature pig. While I was passing the other day, I noticed the mini donk mowing down one of the miniature horses. Full out charged it, teeth bared. The mini horse ran away.
Same thing is happening in your situation. The donky is charging, you're yeilding and moving. Not sure if you should do this, but I have done this with similar charging animals and it backed them right off.
Some might disagree with this, but I hold my ground rather firmly with a dressage whip. I let them know I have it (keep it in plain view), and they would decide whether it was worth charging.
If they charged, they'd get a good beating and be sent scrambling back. Some would rethink the charge, not wanting to chance the whip. One or two have actually charged at me anyways, and I let them have it.
Usually once is enough to make them rethink the action. Now I am the lead horse, and you are the lesser herd mate. I sent you away, you don't send me away.
I'm sure there are other ways of doing this. I never really worried for my personal safety because I had a mind set that I was going to go in there and win that fight. But be warned, a charging animal is dangerous and its up to you on whether you want to face this animal the way I've always faced them.
Once you've sent her back peddling, keep moving her. Move her forward from behind. Don't run her around, just walk behind her and wiggle your whip and click at her if she looks at something else or drops her nose down to sniff at something. Just move her at the walk. Raise the pressure if you have to and then drop it immediately after she gives you what you want (walking forward, away from you). YOU chose when to make her stop.
This isn't a highspeed chase. You are simply walking behind her (out of kicking range) and making her walk forward. Again, make sure her attention is on you and she doesn't stop on her own. Then suddenly stop putting the pressure on. Just stop walking and back off. By this point, she should stop and turn to face you.
Whatever way you chose, never go into that run-in unarmed. Take some form of protection with you. A dressage whip. A lunge whip. A cattle prod.
Just joking on the last one.
And those who were seen dancing were thought to be insane by those who could not hear the music