I once had a young driver speed up, start beeping his horn, swerve TOWARDS me and throw a beer can out the window at my horse, when riding down a small, local street on a young horse. Luckily said horse was fairly quiet and leapt sideways with an attempt to take off, but settled quickly.
I've also had a horse panic on the road at a motorbike, and run backwards into the middle of a main road around a blind corner.
As someone said above, you will need to control your own fear before trying to tackle it with the horse.
Have you noticed that on the days that you're feeling particularly nervous, the horse is particularly worried about the road? If you are feeling frightened before you even get out there, you're going to be shallow breathing, gripping and giving your horse mixed signals. Even the quietest of horses will get worried when the rider acts like this.
Why not go out on the road for a walk by yourself a few times. Walk down the road, and along the trail for a while each day. Then get on your horse and bring a friend/parent to walk along with you. Often having someone on the ground can give you some more confidence to get on and go.
Try doing an exercise along the road side to concentrate on. Leg yield down the road, say 15m, then change direction and leg yield on the other rein. Ride lots of walk-halt-walk transitions, and think about your breathing. Constant deep breaths through your mouth, out through your nose. Make your body go floppy.
Attach a neck strap or monkey strap to your horse/saddle to give you something to grab onto if you feel nervous - then you're not grabbing the horse's mouth, making him nervous too.