Ok, so... you aren't telling us what this "accident" was? How are we supposed to help you if you won't tell us why she's supposedly scarred for life?
Anyway, I think I see a bigger issue: your horse doesn't know how to be social anymore. There's one easy solution: toss her out into a large herd on a bunch of land. She doesn't know how to communicate properly with other horses. If you never let her figure it out, then she'll never learn.
My friend's horse was a school horse for 17 years. He never once got turned out with other horses. Now that she has him, he's out in the herd and he really has no idea how to interact with other horses. He'll go up trying to make friends, and the other horse will threaten him, and because he doesn't realize he's being threatened, he gets kicked.
Your horse might end up like that if you don't allow her to be social with the other horses. Yeah, she might be scared at first, but so what? She's a horse. She'll get over it.
And I think you might be portraying your own emotions over her. She's a horse. Are you sure she's actually scared of the other horse (whom she's turned out with and has no problems with), or is she just feeling you tensing, and becoming unsure of herself? Or is she just being a brat and walking all over you?
I think she can handle walking next to another horse.