I was told to hum. Unfortunately, at that point all I can remember is the melody to "I'll be home for Christmas." Try that in July.Welcome! My horse was like that too, and I only had corn fields to ride him in. People who know him now don't believe me, but he was ca-raaazy at first! When we got to the fields all he wanted to do was RUN, or spook at just about anything (the HUGE 18 wheelers, psh.. that can that's half hidden in dirt- OMG IT'S GOING TO ATTACK!!) He sounds like your horse, one day I did a small trail ride (1-2 miles tops) but he was so worked up he came back drenched.
What I did was keep his mind occupied on the trail. If he started to get worried or eye something funny, I would give him a job to do. I made sure to keep everything slow to relax him.
Though, one of the biggest things that worked for us, was me singing to him. I found I would tense up expecting him to act up, and he would read that and do just as I predicted! But when I would sing to him, it relaxed me which in turn would help relax him. I got really into it at some points, all the barn neighbors who saw us riding down the road thought I was loco!
Now four years later, he's my favorite trail horse who I can bring complete beginners out on! I can take him in open fields and walk through, or if I wish have a controlled gallop through. Except on his frisky days out by himself, he doesn't spook. After so many very long trails, my horse started to appreciate going slow rather than want to run the entire time.