If trained to respect you then being the pasture will not be a problem at all. Yes, even well trained horses will sometimes get a wild hair and start running around when you are in a pasture, so you always should keep an eye out, but if you are doing that you should be able to be in the pasture and be perfectly safe.
I would start with making her respect you with feeding time. I will not usually discipline another persons horse, but if one of the horses in my barn gets in my space while I'm feeding I let them know on no uncertain terms that that is not acceptable. What I did with my mare, is teach her that she would not get her food until she would stand a few steps back and wait for me to walk away from it. This will not be easy to teach two horses at once if they are loose with each other, so if you can separate the mare while you feed the filly, that will make your life easier.
Take a lunge whip with you and make her stay a few steps back from the food. When she tries to crowd in, crack the whip and run her off. Be careful, because she will probably kick out. Continue to chase her away from the food until she will stand relaxed a few steps away, only then is it ok to move off and let her eat. This will probably take a long time at first, but as time goes on she will learn that if she just waits at a respectful distance, then she will get the food faster. If my horse starts to crowd me for her grain now, all I have to do is point and tell her to get back and she will wait for me to set her bowl down and move off before eating.
been taking the lunge whip out with me and keeping all 3 horses away while i set out feed ---- been swinging it around me keeping them 10-15 feet away and when they turn in and step towards me i snap the whip in front of their faces until they turn away
yesterday morning was a defining moment -- i was out gathering the buckets after the morning feeding session and theyed start crowding me and the filly turned around and kicked at me just barely missing my shoulder ---
i chased the crap out of her with that whip trying like hell to catch her and attack her flanks -- i figure i ran about a mile (almost killed me - i'm too big and old to run like that) i kept the filly seperated from the other 2 for half an hour trying to keep her from finding comfort and safety with the mare she thinks is her mother
after that i went out with a lunge line and lunge whip and had her walking circles for me and making her respect my space --- took her a while to understand not to turn in on me --- i think i did about three 30 minute sessions like that -- all walking and no running unless she turned in on me and then i stepped up the pressure to turn her out and away from me
but both her and the mare have issues when it comes to food --- they both want to crowd --- so no one is allowed to feed them except me ---- like i said -- i keep them 10-15 feet away from me with the lunge whip and i will not put up with any crap from them --- they are too big and too dangerous without discipline