A year ago I bought an Arab gelding who is a beautiful ride apart from his problem with traffic.
We are so lucky being surrounded by country lanes, which we hack out on, but its not enjoyable because as soon as we encounter fast moving cars, vans and lorries, he jumps sideways, but he'll go past anything that is very slow moving or stationary.
I have tried riding him forwards through the problems but this seems to make him worse and he ignores my leg aids.
Everyone is telling me that it is my fault and I am the cause of his problem, but I have tried a more experienced rider on him and he is just the same with them.
At the time of buying him, I rode him in a sand area which had no roads or traffic near by and the previous owner didn't mention he had a fear of traffic.
I love my horse very much and he is great about everything else, water, bin liners, even the odd fast moving rabbit - I couldn't wish for more! But these encounters with traffic are causing me to loose my trust in him and my confidence is suffering.
Nicola F - Chester
A. Dear Nicola coping with traffic has become a big part of our riding, especially as our roads are getting busier, unfortunately traffic problems with horses can be the hardest to solve.
In instances when horses have such a negative reaction it is some times better to go right back to the start as if your horse was a
complete novice again. Go hacking with a friend who is competent and confident enough to put herself between you and the on coming
traffic. Hopefully your horse will feel less threatened and can slowly start building his confidence when these situations occur.
Both you and your horse need to take things very gently and slowly and start building your relationship when out on the roads. 
Another good way of doing this and start enjoying your hacking again is to box him somewhere quiet for a few rides.
Some hacks without the worry of
traffic would help him get his confidence back allowing you to enjoy yourself more. 
Stop blaming yourself for your horses problem; it sounds as if this was already an issue before you bought him.