You do have a challenge here.
Try clicker training. It may take you a while. There is a book out on clicker training with horses, I'd tell you the title but I lent mine out.
If you don't want to do clicker training it may take longer. If you can catch her in the pasture start of by catching her, brushing her gently, giving her one treat (for letting you brush her) and let her go. For this to work you MUST NOT let her go if she is objecting to the brushing, stop brushing immediately, let her calm down, ask for something minor (moving a foot?), give her a treat then let her go. This will give you both time to establish a bond of pleasant experiences without added stress. This works best if it is done daily at the same time of day. She will end up soon EXPECTING you, your gentle brushing and your treats. After a while you will probably end up in the "friendly" catagory, and she may even start coming to you in the field (so she does not have to share your attention with the other horses.) Then you can start work on walking away from the field. You may have to do this one step at a time at first (horse moves one hoof heading away from the field), reward, and immediately return her to the field and lead her around the field a bit (this is "work") before you release her. The next day aim for her moving two hooves away from the field, etc..
All this will take PATIENCE. It will take TIME (days, weeks, months.) It will take CONSISTENCY. You will be teaching her that being caught is pleasant, that you can be just as good friend to her as any horse, that you will recognize and reward ANY move to obeying your desires, and that if she leaves the herd she will get to come back and rejoin it.
Too bad she wasn't raced. She'd have a LOT more handling on her.
I'm pretty sure that clicker training would be a lot quicker, but get the book and read it before you start.
Presumably you can get a halter on her (happily?). Does she lead okay in her pasture? if so, I would lead her, and as Jackie suggests, brush, introduce gradually everything she needs. No reason why you can't tack up in a pasture. But really she needs ground training before you ride. She has to be comfortable with your little rituals, and your handling (then tack) and leading and a little lunging, long before she will trust you for riding. She's greenbroke? If she hasn't been ridden much you are basically starting from scratch because she doesn't know your ways. I usually tack my guy up with a halter and lead, I don't do it in a set location because I don't have a barn. so he gets tacked up in hand, wherever we happen to be. It did take time to get him used to the process I think he'd had bad fitting saddles, but he's pretty good now, although I go slow if it's a new saddle. Anything new, he needs a bit of TLC. But way less nervous than he used to be. it just takes time getting them used to everything, just don't overwhelm her with new stuff.
I'd second the clicker training. It can make things so much easier and faster, and I believe it leads to more consistent responses from horses (and they learn to like you). And, you don't treat forever! Classic operant conditioning.
I would suggest trying to move her feet by rocking her back and forth. Most people think you arent accomplishing anything if you don't go 'forward' but if you get her to move her feet in ANY direction you are getting something done! If you get RFD tv, Clinton Anderson is a wonderful teacher and he has many dvds on his website and also many videos on youtube. Alot of times horses are just being disrespectful and the longer you allow her to not move the worse it will get. Good luck, I am sure this is frustrating! :)
You need some professional help! Once a horse figures out they can get away with stuff they never forget!
I suggest you try clicker training and for that your need professional help. It will help you to understand her, and your horse will go wherever you ask.
You need some professional help!