I take you have tried to desensitize her to the scariness of trails. Have you tried sending her out, a lot, with a brave rider who wouldn't be too bothered if she had a total meltdown on the trail and would project an attitude of "whatever" to her?
My current horse (Gypsum) and the one I owned before her (Angie) could both be difficult on the trail when I first got them. Angie would plant her feet and simply not move no matter what you did, or try to wheel around. Gypsum would audition for the Spanish Riding School. When I sold Angie after four and a half years of consistent work, on trail among other things, she had become an awesome little trail horse who would go out alone, lead a group, chase cows, anything. The woman who bought her had been in the market for something fairly unproblematic and straightforward to take out on easygoing trail rides, and my neurotic little quarter horse had become that kind of horse. Amazing. Gypsum has also become an awesome trail horse, who will go out alone, with others, lead a group, go at any gait, etc., (not so great with the cows, though we're improving). It took me about three years to get her to the point of being unlikely to have a trail meltdown. When I first got her, she was (and still is) awesome as a dressage horse and it would have been easy to say, "She doesn't like trails." The first time she had an epic freak-out on one was shortly after I bought her; we rode over a ridge and when we got to the crest of the ridge, we had an amazing view of the mountains and the city of Boulder below them. The sight of all this space totally wigged her out and we started crow-hopping, rearing, cantering in place, all on this little narrow ridge which was a little bit not fun. About five or six years later, we were leading trail rides of small children on school horses over these same trails.
The point is, even a horse who seems like it's going to be an utter lunatic on the trail can improve with time and mileage. I've known only a handful -- I could count them on one hand -- of horses who were never able to handle it. So ask Claire to take DiDi out, maybe? Even on wee short ones until she gets the confidence to go out for a bit longer. At the very least, it's great cross-training for a dressage horse, as they get to stretch their muscles and use them in a different way than happens with collection in an arena. In my experience, that sort of thing keeps them sounder for longer.