we have a pony at my barn who we believe was abused and he can be very hard to catch specially when he first arrived, when I got out to grab him I just take a bucket of grain and sit in the run in shed and let him eat from the grain and when he does I just let him check out my hand and I give him more grain and just further it along etc.. Just be patient
What I did to gain trust with my two mustangs was I would take a chair and every day sit in the pen and read so that they would get used to me, I would slowly put my chair closer and closer everyday. Another thing was I would take a bucket of feed and set it in front of me so that they would slowly come up to me to eat.
She first has to accept your presence and you will notice how close you can get before she is uncomfortable.
Play with that boundary.
Also it may sound brutal but only let her drink from a bucket you give her for a couple days, ( if you can go there's often) she will need to drink and you will offer her something good so she will be less afraid of you.
Generally if you don't have much experience with wild or undomesticated horses I would suggest to seek some help.
If you have Parelli Instructors in the area, they can teach you so next time you can do it yourself.
I would say a lot of patience is a good thing for starters. Your horse can't start trusting you overnight so you'll have to work on it. I would say try and spend time near him but in no position that he feels treatened in any way. He has to come to you and accept your presence for a start. Carrots and treats can be helpful but you don't want to create a habbit out of it, like every time you see him you give him one because he'll eventually get mad the time you don't give him one. When that steps done, you can work your way up to patting him, playing with him, longing him and maybe ride him one day. Good luck :)
Watch at YouTube videos from Martin Ochotecco. And videos from Monty Roberts.
A good idea is sit in their box or paddock with carrots or something like that near to you and wait the horse came near to you. In your case, I think you have to wait the "permission" of your horse to get near to him.
I would say spend time with her, get closer and closer again, and stay until she accepts your presence. And do it many times, make your presence pleasant for her, with food (I'm sure she'll love something you just have to find her weakness!) and go back. I think the most important is being patient! :)
Unfortunately she is a wild mustang so food doesent really work but I'll try turning my back (I have my dogs to keep me safe luckily is she does anything crazy)
Probably time and bribing with food! My horse was like that when I was first around her. It's kind of weird, but it seems like turning your back or crouching down (if it is safe) helps them be less intimidated, so those might be things to try. Good luck! :)