My first thoughts were to get her out on the trail with some experienced horses and I'm still working on someone to go with.
This is a very, very good idea. There is a world of difference between a horse being alone on the trail and being in a group.
We do a lot of riding that takes us through suburban areas as well as on/off trails and spend a lot of time with the young ones getting them used to this environment. In suburbia, there are hundreds of horse monsters lurking everywhere. Just some thoughts from my experiences...
- Although round pen/at the farm desensitizing doesn't hurt, it will never replace miles and miles of on the trail practice and experience. Don't expect your mare to be ralaxed on the trail just because you've desensitized at the barn.
- Horses hate 'change'. When you start out, take the exact
same route every time
until she starts to relax. After that, slowly
expand/change the route, but continue to use the original route. Your goal is to expand her comfort zone and trust, and if you're only able to add a couple hundred feet, that's fine. It will get faster and easier to expand the zone with time.
- As mentioned above, a good seasoned buddy won't eliminate the discomfort completely, but will drastically reduce it. We always start by ponying (or riding) our young ones with our seasoned go-anywhere mare.
- Many will disagree, but IMHO it's OK to get off and lead in hand when you need to... it's less for your horse to think about. Just don't let your horse associate seeing a 'horse monster' with you getting off. Keep her mind busy for a few minutes by turning, circles, just looking, etc and then get off to lead her.
- We always talk of working on getting our horses to trust us. Trust is only gained by hundreds of miles of riding experience....there is no shortcut, and, when riding alone, it takes a very long time
for that trust to come even close to the security she gets from her herd. Remember, this is very
unnatural for a horse and those 'stay safe' instincts are very strong.
- Always, always end on a good note doing something easy and familiar.
Bottom line...practice, miles, experience, age, time, and patience.