I agree with the previous suggestions - when you do go out, go with a small group of more experienced horses. They can really be a confidence-booster for your own horse. Make sure the people you are riding with know that your horse does not have much trail experience, and let them know your goal for now is just to get him used to things.
Adding to that, as the ride progresses, take turns with your "place" in the group. Ride in the middle and rear of the group, and try leading for a bit.
Also, see if you can set up some little obstacles in an arena, especially things like tarps or "spooky" items. Don't over-do it, but it's good practice.
Remember that some horses are very reactive and sensitive to the way their rider responds to things. So if you "ball up" every time you encounter something that you think may scare the horse, the horse will determine that whatever it is it's worth being scared of.
I've seen some people who have managed to turn their horses into quivering, neurotic messes because they (the rider) were so tense about all the things their horse MIGHT be afraid of.
"Parelli horsemanship is just like painting by the numbers. You need absolutely no skill. You just put this color here and this color there, and when you're done, you have ... a mess no one wants." mp