You said the mare recently received ground training, but you didn't mention whether she was broke to ride or not before then so I'm going to have to guess a bit in order to answer your question.
Your mare may be acting up when ridden on the trail because of one of the following reasons:
- She was recently broke to ride and has not been ridden around other horses.
- She's a very timid mare and other horses make her nervous.
- She's been kicked, injured or otherwise hurt by another horse and she's scared of having them close to her.
Go back to riding in an arena or small field. Pick a friend with a calm, quiet, and non-dominant horse to ride with you. Start off by having your horse stand in the center of the field or arena while your friend walks around the ring/field. Instruct your friend to give your mare plenty of space. If you your mare is nervous, dismount and stand next to her until she quiets and settles down. Pet her and reward her for quiet behavior and then mount up. Ask her to continue standing in the center of the ring for a minute or two. If she's quiet and calm, then go to the rail and ride with your friend on the opposite side of the arena. Over time, let your friend get closer to you. (Never allow your friend to get closer than a horse's length from your mare).
The first ride or two, keep your mare and your friend's horse at a walk. As long as your mare is quiet, reward her with petting. If she gets upset, ask your friend to stand still and put your mare to work trotting or cantering - ask her to leg yield, sidepass, circle, etc. until she's got her mind on you. When she does, let her go back to a walk. Make the right behavior (being quiet with another horse in the field/ring) easy by letting her walk. Make the wrong behavior (getting upset) harder by making her work harder when she's not listening.
If she handles the first ride at the walk well, then have her and the other horse take turns trotting on the second ride. Again, give her some space but let the other horse drift closer and closer to her. Over the next few rides, you can let her trot and canter more.
When she's doing well with one horse in the ring or field at a walk, trot and canter, add another horse and over time add several more horses. Once she's comfortable in the ring with several horses, you can venture back out on the trail.
When you start back to trail riding, go with just a couple of friends who have helped you out in the arena. Your first ride should be at a walk only, and your friends need to give your mare space so she won't feel crowded and get nervous. If your mare stops listening to you, do circles on the trail, leg yield back and forth or side-pass and make her work. When she's quiet, pet her and give her a loose and relaxed rein.
Over time, build back up to faster trail rides with more riders. Just remind riders to give your horse space to relax and make trail riding a good experience for her.
Good luck! Soon you should be enjoying trail rides - with a horse who enjoys them, too!