Fear of Riding on the Trail HELP NEEDED!!!

I am a teenager who has been riding for almost a year now. My riding teacher and I ride on trails and up and down small hills. On one hill there was a small fence and my horse spooked, I normally don't ride that horse and it was my first time. So anyway she spooked and almost flew but I held on to the mane and then I was basically sitting on the horses neck. This freaked me out and I started to get really scared to ride on trails. I rode on a pony that was very gentle and didn't spook and thus I regained confidence on the trail when I was riding that specific horse. Here comes my problem! We have pony who is pretty young and wants to do anything he wants. On the trail you always have to pay attention to him or he will start cantering and try to get in front of the leader on the trail. Sometimes he does mini-buks and I am really, really scared to ride him on the trail because I never know what he wants to do. He is extremley strong and wants to always bite the tail of the horse in front of him. I rode him on the trail yesterday and it was ok, sometimes he bucked a little and wanted to bite. I am still scared and whenever I know I am going to ride him on the trail I start sweating and my heart rate goes up like 100...
In the ring I have no problem riding him because it is a closed space and usually there is no horse in front of me.
I am so scared and I don't know what to do to regain trust in my pony!
Any honest advice would be appreciated.

Does the trainer own these horses, or do you?
I would suggest spending as much time bonding with the horse on the ground as you can. That's how I've found to get mutual respect and build trust. You spend enough time with your horse, and you end up trusting them to take care of you on a trail, and he/she feels the same toward you. Trust me, I've been battling fear, too, but truly knowing my horse makes all the difference.

Mistake, accidently posted

You might try a western or australian style saddle, if you aren't using one already. They tend to be deeper and easier to stay in when things get wild. Since switching saddles from English to Australian, I've found my posture has improved dramatically and so has my confidence that I can ride out most of what my horses are going to toss my way. For trail riding, that is critical.

It may be a crutch, but when one needs a crutch, it isn't bad to use one...

Thank you very much for the advice, I will try to spend more time with him and today I pulled his mane and showed him who the boss is and slowly but surely I think I well get my confidence back
Thank You!

I know how you feel .. recently my horse spooked and then bucked me hardly and it
Hurted me so much , and I was afraid to get on him again , but my instructor made me do it anyways , so what I did is , I rode the horse I usualy ride , which he is very gentle and warm , and then I would get into my horse , and ride him just into trot and walk , and some ground work , I didn't rush my self , the weird thing is .. I jump
Over a meter , with any horse I could ride , but when i'm riding him , I got this fear that i'm going to get bucked .. so as I said I started thing slowly , I recently started to canter with him again , oh and I changed the saddle he seems much calmer now , i'd say spend much time with him just try to bond =)

Good luck for both of us ;)