First of all, always ride her in your arena first until she's a bit tired and bored before taking her out. Also, it might help to do some more groundwork with her to reinforce habits of obedience before you try to ride outside the arena, making sure she will obey voice commands to walk, trot and whoa. That way you can use your voice to keep her moving forward while legs and reins keep her going in the desired direction.
Can you get someone to ride her pasture mate out on the trails with you a few times so she feels she has some moral support while facing the bogeymen in the strange territory? And then once she's accustomed to going on the trails take her alone, but lead her at first (as explained next).
If you can't manage to get a companion to help, then start by leading her around the trails by herself. This way she sees you beside her and may figure if there are any man eaters lurking in the bushes they'll eat you first. Go a bit farther each day, praising her and rewarding her with a treat or two along the way when she's being particularly well-behaved and always when you've gone as far as you intend to for that day.
Once she's no longer fighting to stay with or to return to her chum lead her out, then mount and ride her back. When she's handling that okay, mount part way out and continue away from her pasture for a bit before returning home. Take it slow and easy, remember the rewards, and don't be discouraged by occasional setbacks.
Another way to work with this is to ride a lot where she will go. Make it work to stay near her pasture mate. Then, periodically attempt to ride her away from the mare. Try to turn her back around before she gets cranky about it. Make "at the barn" work and leaving relaxation. Plus the constant to and from will let her see she isn't going to just "leave" her buddy forever. Just be sure you are working her near her buddy where she wants to be, and allowing her to relax as much as possible going away.