Oh please do not find a willing teenager to take this on.
Convince your horse that where shes trying to go (back to the barn) is more work than the trail. I agree 100% that this rearing is not something you ride through, I’ve been here with a dangerously anxious horse.
Once you realize the horse is only trying to go where there is no pressure, it will be a whole lot easier. Of course she wants to go back to the barn… that’s where her friends, food and no pressure is. People climb on, take a hold of their horses mouths and generally try to just push them through stuff when they are uneasy or anxious about things. Take it back a step or 5.
Assuming you’ve covered the ground work with sensitizing, desensitizing, and lateral work… start in the arena.
Is there any suggestion from the horse that shed like to head to the gate? Work on that, no reins/direction from you. Let her go where she wants, and when she heads back to the gate, make her work/disengage her hind end. When you finally feel her bulge her shoulder away from the gate, allow her to walk off on a loopy rein. When she circles back around again, repeat. As many times as it takes for her to decide that the far end of the arena is a good spot. When she does venture to the far end on her own, let her stop and rest there. Make sure that your rest spots have previously been undesirable to her.
Then take the same process outside, allow her to head back to the barn when she wants to. Then work her there. Repeat as necessary, until she willingly heads out on a loopy rein.
You’re going to change her mind on where she’d like to be, and you’re going to allow her to make mistakes in the process.