i know what you are going though, and the best thing is to go back to basics. Grab a nice brush and give your horse a massage. Go for a walk with him. Maybe get to know him a bit better, and let him know you. You need to be very patient and show him that you can be a leader, and can earn his confidence. This will not happen overnight, and will take some time. The thing i think that you are having a problem with is not trusting the horse, which because he doesn't trust you is just waiting for an accident. You can't magically gain that trust in one day, you'll have to start small, which is why i suggest all the off-saddle stuff. You would also probably greatly benefit from riding another horse first, just so you can get back into it on a horse that doesn't have "baggage." Maybe ask for a western lesson or two, if possible. Unless you're on some sort of deadline to train this horse you probably can afford to go a week or two without riding him, just make sure he's still exercised =) To me it seems like you have a new barn, new horse to deal with, as well as a new discipline, on top of which you're not all that used to riding right now. Just take some time to get used to everything, and let your horse get used to everything.
For riding, i have found singing to help. Also, small circles help. They make you feel more in control, as well as forces the horse to balance. If a horse is turning in small circles he is less likely to speed up or buck/rear, because he is trying to balance himself. Maybe try making a small 10 or 5m circle in each corner. You could also maybe get him used to the arena first, maybe lead him through it or free lounge him.remember to let the horse have some fun too, and let him associate the arena with something good. I don;t know your horse's specific problems, so i'm mostly trying to go by what a younger, green and maybe slightly mistrusting horse would need. I wish you the best of luck, remember to enjoy your horse, you never know how long you have them =)