Trust between horse and rider really comes only from the miles spent in the saddle. Once you've learned all of her reactions to all kinds of things, and she understands you are trustworthy to communicate in a way she understands if she does a little hop or speeds up temporarily, she'll settle more and trust you more.
You can ride a horse and train a horse for endless hours in an arena, but horse and rider also have to spend a lot of time riding together out on the trails to build trust in that different environment.
In general, I've found that I needed to ride a horse out regularly for about three months to get a start on both of us trusting each others' reactions. After about six months, you start to feel pretty comfortable. After a year, you can start feeling like a real team, but it can take even longer with some horses.
On another thread we were discussing how sensitive horses are to us. They read each individual extremely well. Trust from the horse and the response you want will have to come from your communication with the horse; you can't just get it from things a trainer has done with a horse.
You can't program a horse through a trainer to respond a certain way to things because the horse will respond differently with each rider on her back.
If you can respond in a way similar to how the trainer got good results, the horse will soon learn to respond to you in a way similar to how she responded to the trainer. If you respond differently, then the horse will only learn a new way to respond based on your riding. I hope this makes sense.
Many people have their horses trained but never learn to communicate with the horse themselves, so essentially what they get from training is a horse that goes well for the trainer. I can hop on a lot of the horses at my barn and take them for a great ride, but that does their owners no good. The owners have to learn to ride the horse also.
I just feel this post merits repeating.
It's nothing to do with the horse's age and how long you've spent with her. It's that if you want a horse to go well on trails, you need to ride it on trails consistently and for many months before you become a real trails team - no matter how good an arena team you already are.
If you're not comfortable riding on trails, that adds to the situation, as a horse will often pick up and amplify on any
lack of ease on the part of the rider (which is often anxiety about what the horse will do). Catch-22? The good news is, as you both ride more trails, you'll both get more comfortable. "Conveyor belt" is an excellent prescription for your situation as well.
What happens when you're riding with a horse whose paces match yours? A lot of highly strung horses get antsy riding with plodders, so riding with a more matched horse at first would be helpful in settling her down and giving both of you confidence. It's removing one of the variables that are making things uneasy. You can re-introduce the "riding with slow company" variable later on, when you've dealt with a few other ones.
Best wishes, persist, and enjoy your trails!